with usb, as far as i know, you can't!

What is the make and model of the soundcards? Electrically speaking, there is no problem about plugging two sound cards into separate USB ports on your host computer.

You will encounter at least two operational problems:

(1) Your audio software and/or device drivers may not be able to handle two separate (and different?) interfaces.

(2) You have to synchronise the two interfaces so that they sample the input waveforms at exactly the same instants. If the interfaces are equipped with wordclock, S/PDIF or ADAT inputs and outputs, this is not difficult to do, but I am guessing from your post that your interfaces do not have these facilities.

OK, I thought you had a soundcard but in fact I now believe you have this unit. Is this correct?
http://www.sweetwater.com/store/detail/US122L/

Which is not a sound carrd but is defined as an "interface" which is different than a soundcard which plugs into a slot inside your computer. This unit does not daisy chain. Some of these small interfaces do.

So if you want to track something like drums and do so on a regular basis then the purchase of a more versatile interface would be my recommendation, something with at least 4 but probably 8 channels like these
http://www.sweetwater.com/store/detail/KonnektStu48 a highly recommended unit
or what I use though not as good as above
http://www.sweetwater.com/store/detail/KonnektStu48
or if you want to stay Tascam
http://www.sweetwater.com/store/detail/US1641/

One of the issues with the Tascam unit is the connection to the computer is USB rather than Firewire and USB is not recommended above two channels by most serious engineers. Now if you are going to purchase a Mac you need to look into whether it has Firewire some models don't.

There is a way you can use the unit you have which is by adding a mixer into the equation which basically you take the four signal sources and mix to stereo and send the stereo signal left and right into the two channels of the tascam. The drawback is you are commited to the mix you make at the board and it greatly limits your ability to tweak the tracks later when compared to getting 4 seperate tracks for each mic. IMO you would be much, much better off with a larger interface now and down the road if you are seriously thinking about recording.

Easy. Create an aggregate audio device.

Try this for help:
http://www.apple.com/pro/techniques/aggregateaudio/

good link!

good link!