For posterity's sake, and to avoid people buying lots of unnecessary 99c USB audio devices...
...If you're running Windows Vista, 7, or 8, and your audio driver provider sets up the audio device as multi-streaming you can use multiple (in/out) ports at once. Note that this is a function of hardware and configuration by the driver, so it may not be available for you if your hardware doesn't support it.
If you right-click the speaker in the task tray and select 'recording devices' you'll see a panel pop-up. If you see TWO microphones listed there, chances are good you have multi-streaming enabled.
Some manufacturers will configure the audio device to not show this--and instead will set it up with minimum number of audio end points--because having more end points makes it more confusing to the average end user (increases the number of support calls). However, Microsoft has a requirement that audio device manufacturers offer two inputs and two outputs to pass their WHQL tests with the Microsoft audio driver, so often with 2007 and later PC's if you uninstall your manufacturers audio device and use the built in "Microsoft High Definition Audio Driver" you may see the add'l inputs available.
If the PC is older, 2005 or earlier, it could be AC'97, back then it was common to use a "wrap back" system where inserting a device into the front panel would literally break the connection to the rear panel, so it was physically impossible to use rear panel when front was in use... but since the swap to High Definition Audio in 2005/2006 that is no longer accepted by Microsoft. They want each port to have it's own connection to the audio chip.
...now Scannercast would need to understand independent audio end points for that to work, but based on the screen shot someone posted earlier in the thread I'd say that it probably does.