If the satellite is running HD, it won't work. The feed coming in for satellite is to high compared to cable and will cancel the cable feed out. I tried doing this for a customer a couple years ago. I talked with Spawn USA who does the di-plexers for Dish and DirecTV, and that is what they told me after I tried merging the 2. Ultimately, you'll have to run them on seperate lines.
This is CABL.com posting #333050. Tiny Link: cabl.co/mbyNW
Posted in reply to: Triplexers?? by thesatelliteguy