Think of a diplexer as a "combiner" of sorts. It combines the satellite signal from the dish along with an RF feed (such as cable TV or off-air antenna) and feeds both signals along just one
coaxial cable. This eliminates the need to run a dedicated coax for the satellite if a cable already runs into the room where the Dish receiver is. Very handy if it is physically impossible or difficult to run another cable to that room.
Dilpexers can
also be used to "back feed" the RF output from the Dish receiver down the incoming cable and feed it to another cable going to another room. In this situation the two signals are travelling in opposite directions. I suspect this is what your
installer did since you mention UHF capable Dish receivers. In this setup you can watch the Dish receiver on several TVs since you have a UHF remote.
Its hard to tell how your wiring is setup but remember that diplexers are always used in pairs -- the first diplexer "combines" the satellite signal and the RF signal, and the second diplexer "uncombines" the signal into the separate feeds. So its a matter of identifying which cables are for what and connect the diplexers accordingly.
kat