OK, so you are planning on keeping the 5100, I assume?
Then yes, install the DPP Twin. Run 1 line to the 5100. Run the other line into the seperator, then the 2 lines from it into the 9200. Let me know if this isn't clear; and I can put together one of my diagrams tonight when I get home. (I can't get to Photobucket from work.)
Now, with the 21" dish- could it be 20"? I ask because Bell doesn't make a 21" dish. What I am really looking for is this: is there a "Y" adapter for 2 LNB's on it?
"There are 10 types of people in this world; those who understand binary and those who don't."