Hi, and welcome!

I'm assuming you are talking about acoustic guitars and recording them using microphones rather than (or as well as) pickups. With both performers playing at the same time, unless you record in separate rooms or using isolation booths you are going to get some cross-bleed between the performers, and each performer will get bleed between his/her vocal and guitar microphones. There are ways of minimizing the problems it produces, but basically you have to live with that and work with it. The techniques needed fall into several categories, and these include: how you select and treat the room to record in, how you place the performers in the room, which microphones you use and how you position each of them relative to the performers and their instruments.

I put the room first in the list because it's probably the most important factor, even for a single performer, but I'm not going to dwell on room problems. If you are trying to reduce bleed between vocals and guitar, the usual microphone technique is to put each sound source in the pattern null of the other's mic. For example, I have used a figure-8 stereo ribbon microphone positioned horizontally in front of the player with one channel facing down towards the guitar and the other facing up towards the mouth. The performer-microphone spacing is arranged so that the microphone subtends a right angle to the two sound sources (guitar and mouth). The result is that the guitar is in the null of the vocal channel's pickup pattern and the vocal is in the null of the guitar mic's pattern, and it generally works well. However, a fig-8 mic has by definition a rear lobe in its pattern, and this is going to catch room acoustic reverb and floor/ceiling reflections, and this would be before you even start to think about other sound sources such as another performer.

If you use so-called unidirectional microphones (e.g. cardioid or hypercardioid patterns), you can reduce the amount of room reflections and second performer getting into the track. However, because the microphone pattern nulls are much further to the rear of the mics, it's more of a problem to sort out an effective pair of positions for each performer that give an acceptable main sound combined with low bleed from the accompanying instrument. This gives you two options: placement so the secondary sound sources are still in the pattern nulls, or close miking to gain an amplitude advantage over secondary sound sources. I think I would start with close miking for the vocals and careful positioning for the guitar mic. This would be difficult for a straight cardioid like a Shure SM57, but the nulls of a hypercardioid such as the Beta 57A are at 120 degrees, and so it's quite reasonable to position a Beta 57A as the guitar mic facing 30 degrees down from horizontal and still maintain a standard distance of about 12" - 15" from the instrument body. Doing it this way, I would place the performers on thick rugs to reduce floor reflections.

All this assumes that you have an audio interface with at least 4 microphone channels. Before I fill another page, perhaps you had better come back to us saying what you already have, what you are planning on getting and an idea of budget. We can take it further from there.

I rarely worry about interactivity between microphones. This is a common function of all live recording. And this interactivity a.k.a. bleed can actually work well. It was a bigger factor back in the day when you're spending + $300 per hour at a commercial studio. At the home, after all, you're not dealing with that kind of budgetary problems. Having this extreme need for isolation can actually be detrimental to the performance sound. Bleed can be your friend acoustically. Most dynamic microphones are less sensitive than their condenser counterparts and why I frequently recommend SHURE SM58/57's. They create far less problems than condenser microphones do. And they can sound every bit as good as a + $3000 German condenser microphone and I'm not kidding.

Recording two guitarists and two vocalists would really require a $500 minimum interface that can accommodate 8 XLR microphone inputs. A pair of microphones for each guitar and a vocal microphone for each vocalist. So you're looking at a minimum of six inputs at once. Microphones working altogether can have some phase cancellation anomalies when used too closely together. You must also rely on the microphones polar pattern of directionality. So being next to each other would be more problematic than facing each other. This would put the microphones pattern facing the opposite performer in its best null position. Using common furniture elements at home can also be utilized to provide many acoustical advantages of some isolation. Heavily padded chairs, couches, tall dressers, bookcases work well in these instances. And their greater mass provides for better or low frequency absorption then just a few pieces of foam stuck around the room. Items on the wall that are hung can also provide for some diffusion. Parallel walls with nothing on them can be quite problematic. So placing things on the walls and in front of the walls helps to create diffusion. It's amazing the sound you can get without having to spend one red cent. Couch cushions, bed mattresses are all beneficial and then can easily put back into their proper places after your recording. I don't even get these luxuries on live on location recording environments. That's where careful microphone selection rather than worrying about overall fidelity is more important. Dynamic microphones are much more effective in a home environment than our condenser microphones. So even those ubiquitous SHURE SM58's can sound much better than anyone's condenser microphone that may cost 10 times as much. And there really is nothing to be lost by the use of these. You utilize a condenser microphone when you absolutely positively have to have that kind of sound. It is not a prerequisite for professional recording however.

As Boswell stated and I will also, knowing what you have available and in use would be much more helpful in helping you.

I like the simple things in life
Mx. Remy Ann David

Hi thanks for the advice people, its been really helpful. Going from this information and lots of google-ing and listening to other peoples home recordings, we're going to record the guitar parts together at the same time and not worry about bleeding since (as you have said) its hard to be avoided and works with acoustics. We're then going to play the acoustic track back through headphones and record the vocals separately to isolate the vocals for editing (if its needed). Our budget isn't huge so we're going for two condenser mics with pop filters, stands and a 4 input interface with the plan of improving building up the equipment if needed (:

I might also suggest that instead of going for a mediocre condenser microphone or a pair, or four, that you simply try out some SHURE SM57's. Plays well on acoustic guitar they can sound stunningly beautiful. They're extremely rugged and impervious to most damage, whether, humidity, dropping on the floor. Condenser microphones can actually get rather brittle sounding on run-of-the-mill preamps. They can be quite unflattering. Whereas the slightly bandwidth limited SM57's can bring out more balls to the sound of acoustic guitars without getting too strident.

Condenser microphones on male vocals frequently work well. But even the venerable SM57/58 along with some extra foam pop filtering can compare nicely with the + $3000 US Neumann U87. Who could want anything else? Everything else is just going to be a bad imitation of an 87. A 57/58 will get you closer to an 87 than a bad Chinese condenser knockoff. Though if you shell out $500 + you can get a nice condenser microphone. Are you rolling in cash? If so, go for it. You might even want to look into a couple of ribbon microphones such as the Royer's, SHURE, Beyer, A E A, Cloud, Cascades which all sound really luscious on acoustic guitar. Really smooth on vocals. None of that typical harshness that has become so prevalent and which I find hard to listen to comfortably. Nothing else sounds like a ribbon. They do not respond to the pressure they respond to the velocity of sound. That's why they are also known as velocity microphones. And they typically have a figure of 8 pattern but many have hyper cardioid polar patterns as well such as Beyer M-160's. They are quite tasty. About $600 US each. I had four until one got smashed. The thought of that still stings. Killed at the National Press club, Washington, DC.

M-160 #4 RIP
Mx. Remy Ann David

I think we're pretty settled on the gear we're getting as it fits our budget perfectly but thanks for the advice :) and we've listen to other peoples un-edited recordings with them, we will probably end up recording with pickups and mics for the best sound. We'll probably look into buying a sure sm57/58 when we start recording electric too.