A USB device will generally allow a single stereo pair as output.
In other words, a left and a right. The left is one channel and the right is another.
If you have two microphones attached, you should be able to choose in your software whether to treat the resulting two channels (left and right) as one stereo track or two, separate mono tracks.
In short, it will do what you want.
The difficulty with most USB devices comes when you have more than 2 inputs to it. Say you have 4 mics attached. It will still only allow a single stereo pair out (i.e. two tracks). What is generally done there is that all 4 tracks are summed into the 2.
Firewire devices, on the other hand, will enable you to have 4 separate mono tracks.
Really, for 2 tracks, USB will do what you want in this respect. There are other considerations, but being able to record separately isn't one of them.