Follow-up... here are some of the potential issues I see:
1. RoadRunner might get in the way. I know I can default mplayerc (the video player for RR) to output to a specific sound card. However, I'm worried RR won't allow playing MP3's and videos at the same time. I've got dualmon option turned on for dual monitor support, and it continues to play videos when I exit the video player to use other functions, but as soon as I go into another "media related" function (FM tuner, MP3 player), it stops the video. Maybe when I get a second sound card, this will go away. Otherwise, I'll have to figure out how to tweak RR to work around this issue.
2. A/B switchboxes may get confused when sending them IR signals (i.e. it will switch both switchboxes because they use the same IR signal)... I will have to either a) find switchboxes that have different "channel" settings for the IR commands, b) use different brand switchboxes (which presumably would use different IR codes), or c) use two IR transmitters positioned next to each switch box, but without the possibility of one bleeding over to the other. Option a is preferred... I'll need to make a shopping trip to Best Buy and/or Radio Shack to see what's available.