There's is a BIG difference between the direct cable coax connection and the DirecTV s-video connection on my AIW Radeon 7500.
My advice: Get another receiver (cable OR DirecTV) and use the s-video connection (composite connect at least).
For DirecTV:
If you have ONE dual LNB on the dish, you will also need a multi switch [3 IN / 4 OUT (you will use 2...the 3rd is for local antenna)] to hook up a 3rd and/or 4th box. The additional receiver is $50-$60 and the multi switch is about $30. Plus cable.
You cannot use regular splitters!
* [You only need a multi switch for 3 or 4 receivers(total)-not for only 2 (total)]
You DO need a dual LNB on the dish for two, though... run one line to each box and you're set.
GL
You can only change channels from the cable or directv box- the tuner in the tv card gets set to channel 3 or 4 - like VCR settings.