I
think you can use two soundcards at once with REAPER, but I don't know how to set it up and someone else will have to help you...
The "normal" way of doing it is to get an
audio interface that has two (or more) microphone intputs.
An alternative is a stereo mixer with one mic panned left, and the other panned right. You can plug a mixer's output into line-in on a desktop/tower computer (mic-in on a laptop won't work), or you can get a mixer with a USB connection.
Note that audio interfaces & mixers have XLR inputs for stage/studio mics and they won't work with "computer mics".