If I'm reading you right, you said there was just 1 cable coming down from the dish, and 2 cables going into the house. To me, this sounds like 1 cable is feeding the signal from the dish, and the 2nd is going to the TV2 location somewhere else in the house. So if I were you, I wouldnt worry too much about re-wiring the system cuz there could be diplexers somewhere or splitters or who knows.. If it works, leave it be. Simply install the LNB loaner, connect the 1 cable at the dish to that LNB, disconnect the DishPro seperator behind the receiver and connect the cable direct to the receiver. That'll give you picture on 1 tuner. You won't need the SW21 switch.
of course the original problem might not be the LNB, could be a line of sight issue or a misalignment. If the loaner LNB doesnt work, try the check switch idea scooper suggested. WHen you run checkswitch w/ no cable installed, it resets the receivers switch matrix. Sometimes, not always, if you dont reset it, when running new switch checks, the receivers gets confused

If its out of alignment, an nothings in the Line of sight, you could try a slight adjustment of the dish. The 500s are not hard to align. If you don't wanna mess w/ it, leave it for the pro.
If there are 2 cables coming down from the dish, and 2 going straight into the receiver, then the simple LNB switch should solve the issue, assuming of course the LNB is the original problem.