You may be able to install 2 but probably only record from one at a time. like you could have 2 cams or printers set up and then select which one from the device selection in most software.
I'm not sure if you'll be able to record from both at the same time though... you may have to have some other hardware... I'm no recording expert... but you could even have a mixer that runs a stereo out into your sound card if you already have one and don't want to shell out extra bucks...
my friend uses a firepod and he gets 8 channels via firewire... his is a little unrealiable but it was scratch & dent clearance. he also has the inputs into the firewire coming out of a mixing console so he can put several mics on one channel.
I know they also make mixing boards (berringer i think) that have usb output on them to run directly into your pc.
Or you could probably get a soundcard that you put on there that actually has mutliple inputs.