I think your best bet is to buy a microphone mixer and plug that into your mic or line in. If you do a search for Rolls mic mixers, you will see all kinds of different small, portable mic mixers. Mix the two mics together and record the feed. If you want to get fancy, you can get a stereo mixer. Radio Shack has a $30 mixer that handles 4 mics or 2 mics and a stereo line input.
As for keeping one mic from bleeding over to another mic, you need to do what they do in a recording studio by using sound-absorbing material between the two narrators. You can use foam from an old futon mattress, or pillows, or thick curtains, etc.
Good luck.
