You can reassign more than one video/audio input for each source button, but the higher quality source will always take precedent. Therefore, if you want to use one source button for more than one input, make sure the higher quality unit is turned off when you want to use the lower quality input.
Example: I have my Wii and PS2 hooked up to vcr (PS2 analog (yellow) video to vcr analog video, ps2 optical to optical 3, wii video to component C, wii audio to vcr audio). If both are on at the same time, I will get PS2 optical audio and wii component video.