I bought a Toslink splitter at Altex Electronics for about $7. I used it to hook up my Xbox and my MCE to one optical port on the receiver. It works great. That being said, most of the time its on the MCE box, and the Xbox doesnt sent any optical signal when its off. I dont know if you could do it with two satellite STBs as I dont know if they kill the optical signal when you power it off.
I just checked Altex's website and they didnt have it listed online, but if you search Google for "toslink splitter" a bunch of stuff came up.