Yep, you should be ok. There would only be an issue if the sat box could be set to use only a single ouput, which i doubt. On my cable box, all outputs (digital and analog) are enabled simultaneously and what you propose would work just fine. I assume you want to do this so you can watch sat on the tv without turning on the receiver - correct?
Lots of places sell optical splitters. Radio Shack has a few.