what you are describing as your install at your house is a very simple example of signal stacking, ecspecially now with dish's dual tuner receivers most of us use diplexers on all kinds of back feeds, ota, and cable combination...
the reason i am chiming in is because it just kind of aggravated me that you are sitting here trying to shoot down the advice given by the so called "experts" on this forum... i agree, there are some installers out there that don't know what they are doing... i had a guy working for me that installed these things for over a year, and never saw or used a diplexer... but i think your 9 out of 10 ratio is way off, i think it's probably the other way around..
also, the original post was put on this forum because he wanted the "experts" advice, he got it, DON'T DO IT, it won't work by any easy means anyway... i mean sure, if you are willing to pay any amount possible, i'm sure some electronics engineer out there can figure out a way to drop the freq of one of the signals, then at the inside of the house convert the signal back to it's original freq.
in fact, from what i understand about dish networks proplus technology that's kinda what they do, one side of the separator uses the dishpro freq, the other uses legacy freq... (i could be wrong about that)
anyway, he asked a question, the "experts", (the guys that mess with dishs all day long, 5 or 6 days a week, and are certified to do so) gave him the reasonable answer to his question.... NO, Can't be done