Some help with Quad-Capture

"Quad-Capture sends a stereo signal to my computer even if I am using just 1L (a single input port) with a microphone.  The result is that if I record using something like Moviemaker, I only get audio out of the left ear on my stereo headphones when I playback through the computer "
 
It took me while to figure out what you were saying. This circumstance is probably just a simple limitation of MovieMaker and the way it uses the audio drivers. Just about every Digital Audio Application will record one of the sides of the stereo driver and play it back as a mono stream within the software. In other words, you don't have to use the "mono" switch on the Quad Capture hardware.
 
"Can I use an XLR cable splitter (female to male-male) on the microphone and then just plug into 1L and 2R?  Will this have any negative effect on the phantom power since I am using a single condenser mic."
 
I wouldn't do it. The phantom power is probably not an issue if the cable is wired properly and each channel in the Quad is wired consistently. You'll still have 48v on pins 2 and 3 and pin 1 will be ground but the mic will see a much lower impedance load. If you really want to use Movie Maker and record a mono source as two track then I'd use a small mixer to do the splitting. It will act as an impedance buffer.
 
If you don't have another DAW to use with Movie Maker you might try the free app "Audacity" to record your tracks and use it to convert to two track etc.
 
Good luck.
 
best regards,
mike