OK, so, in summary: You have 2 microphones. One is an ordinary computer
microphone on a headset. The other microphone works through a preamplifier
and goes to "line in" on the sound card.
So you need a sound card on which "line in" and "microphone" are 2 different
jacks.
That is a reasonable request, but I don't know which current sound cards
have that setup. Sound Blaster Live 24 defininitely does not. Sound
Blaster PCI 16 does, but it's probably obsolete by now. I have one of each.
Can others chime in with suggestions? I know that with Sound Blaster
products, you can go to
www.creative.com, under support, and read the
instruction leaflet for each card.