Why don't you keep the onboard stereo mic for one instrument and add the LDC to the same unit ?
Or you could use only 2 mics. If it isn't a show, you could record with spaced pair or Blumlein (if the LDC have figure of 8 patern) or plain x/y.
Then you listen to the result and move the violonnist until they both sound good on the recording...

Ok let's say you can't or you don't want to... Having tracks from 2 recorders to combine offer 1 big challenge : synchronisation...
One good thing is that you have 2 identical recorders. If it's gonna be easy or not depends on how precise their internal time is.
Second challenge is phase. With condensers, you'll certainly get spills/bleeds from an instrument in the mic of the other instrument. If you don't have perfect synch and placement, you'll get some frequencies shift and cancellation. (phases)

Some of RO members do live recording similar to what you want to do.. Let's wait and read what they have to say ;)

in the first place because it's digital you've had your way with the pooch already. but because it is digital i would bet that as long as the bit and sample rates of two identical are set the same and all other variables are also the same you should be able to load them into a DAW and line them up. it would be prudent to do a clap board or something at the beginning of each recording. be sure to watch phase issues and try to keep stereo pairs on the same recorder. in the end it's digital and and not really well done digital (cheap pres / mics) and it will most likely be streamed and played on devices that won't reveal any sins anyway.

The problem is not so much lining up the start instants of the two recordings, but their staying in sync through the piece. Since the sampling clocks of the two recorders will not run at exactly the same average rate and also will drift differently during the course of the performance, you will not have the identical number of samples in the same time period on the two recorders, which will show up as a different length of performance between the pairs of tracks once the two recordings are combined and forced to run at the same clock rate. This length difference may be only 100 ms or so, but it will ruin the listening experience.

You could (in principle) adjust the two to last exactly the same time, but there will be differential variations over the length of the performance due to temperature drift. You might get away with this on a 3-minute pop song, but not with classical pieces that can last over an hour.

The only way to avoid this without (pointlessly) using up a track on each recorder as a sync track is to use recorders that will take an external sampling clock. An example of these is the Alesis HD24: 24 tracks per box expandable to at least 96 tracks by cascading boxes, all tracks recorded in sample-exact sync.

While it's technically true that they will drift over time, being the same make and model of recorder their clocks will likely stay close enough for long enough to be usable. It's not ideal but it's probably acceptable. I just did a live recording with my Zoom H5 and a Sony camcorder, and there was no noticeable drift over the hour long set. I could sync them up* near the start and they were fine at the end.

*Since you're using them to record substantially different sources it would be helpful to do the clap board thing. For my recording there was enough in common between the two tracks that I could zoom in and rough in the timing. Then I panned them to opposite sides and fine tuned by ear, exploiting precedence effect. The camera audio wasn't used in the final so the most important thing was that the video looked right with the audio.

There is a problem here. Stereo. Stereo imaging depends on the relative level and timing of the signals arriving at the mics. Combining recordings, even when synced can wreck these carefully constructed images. I had to listen to thousands of amateur recordings of choirs and orchestras and very often the main recording was stereo using one of the popular techniques, but often in the forlorn hope of making a small, acoustically dead space sound bigger, they'd add rear microphones which they'd try to blend in. Universally destructive results - a characteristic hollow sound caused by comb filtering, and if you viewed the stereo field on an X/Y scope the image you would see would be a busy, messy display with conflicting and constantly changing locational content. Sometimes, I'd get a recording in this room in two mic stereo, and you could hear and see where the players were by the angles displayed on the screen, and closing your eyes gave the same info. The ones with merged stereo would just be a full display with no clear directions. My own view is that nothing beats a single pair of mics in the right place. So if you have two recordings, does either stand up on it's own? If so - use it!

paulears, post: 450641, member: 47782 wrote: There is a problem here. Stereo. Stereo imaging depends on the relative level and timing of the signals arriving at the mics. Combining recordings, even when synced can wreck these carefully constructed images. I had to listen to thousands of amateur recordings of choirs and orchestras and very often the main recording was stereo using one of the popular techniques, but often in the forlorn hope of making a small, acoustically dead space sound bigger, they'd add rear microphones which they'd try to blend in. Universally destructive results - a characteristic hollow sound caused by comb filtering, and if you viewed the stereo field on an X/Y scope the image you would see would be a busy, messy display with conflicting and constantly changing locational content. Sometimes, I'd get a recording in this room in two mic stereo, and you could hear and see where the players were by the angles displayed on the screen, and closing your eyes gave the same info. The ones with merged stereo would just be a full display with no clear directions. My own view is that nothing beats a single pair of mics in the right place. So if you have two recordings, does either stand up on it's own? If so - use it!

This has a lot of sens and goes along with my suggestion of using only 2 mics. Thanks Paulears