I have a 510 and an 811 connected to my HD RPTV. I have a Pioneer PRO510, and if you have sources on 2 separate inputs, PIP is available. Right now, all my sources are going through a video processor, so I can't use PIP.
You will need 2 sat cables, you cannot split the sat signal. In my setup, I only have 2 coax runs, and I need 3 since I also have OTA. You can't use diplexers with 2 sat signals, but you can use them with a sat signal and an OTA signal. I combine the sat and OTA at the multiswitch with a diplexer, and split them at the receiver with another diplexer. So ihave the outputs of the diplexer connected to my 811, and the 2nd coax run to the 510. I then connected component out from the 811 to my processor, and connected S-video from the 510 to the processor (of course, you would connect these to different inputs on your TV if you don't have a video processor).
I also changed the remote address of my 510 since I use a universal IR remote.
PQ on SD are about the same on both, with a slight edge given to the 510.