What makes it impossible to use WMC for both Dish and Comcast? You should be able to setup one tuner for each(quite simple if they're different types of tuner but a bit more work if they're the same).
I'm not sure if there's anything newer/better but I've used firewireSTB in the past for channel changes on STBs.
I'm not sure if there's anything newer/better but I've used firewireSTB in the past for channel changes on STBs.