I don't completely understand what you are trying to acheive, but if you are talking about using diplexers to combine a Sat signal and something like an OTA, then another diplexer near the box to separate the two signals, then yes, but there will be a compromise in signal strength as each time it runs through a diplexer, it suffers a bit each time, but it usually is not a problem for the vast majority of people. The misunderstanding may be in the confusion of the nomenclature regarding terms "combiners" and "splitters."
Iceberg's response is correct if we assume you meant "splitters." Well, a clarification would be nice, at least for me, as to what EXACTLY you are trying to acheive. It could be that others understood you perfectly, and I am not getting it.
Sorry, but the picture provided makes no sense to me. According to the picture and text provided you have absolutely no need for either the combiner nor splitter as all you need to do is run each cable from the switch output to each of the single tuner boxes in the picture. In other words, take the 4 cables you want to run to the "combiner" and, instead, plug each one directly into one of each of the 4 single tuner boxes. DONE! Keep in mind that the single cable from each of the 4 outputs to each of the 4 boxes provides all 3 satellites to the one box. The DP34 is itself a kind of satellite "combiner" and "splitter" already, for the sat signals. Is that the issue?
Take the cable for each satellite and directly connect it to the inputs of the DP34. The DP34 essentially combines up to 3 satellites so that only ONE cable from ONE of the outputs provides access to all 3 satellites. The DP34 essentially "splits" this to 4 outputs, each requiring only one cable to each box. I use the term "essentially" because the switch is not exactly combining and splitting; more is involved.
For dual tuner boxes, you would need to use 2 cables runs from the DP34 switch to make it work, say outputs 1 & 2 of the DP34 and connect them to the box's Satellite 1 & 2 Inputs respectively. A DP44Plus switch requires only a single cable from the switch to feed a dual tuner box, where, just before the dual tuner box's Satellite Inputs, a special Dish Network separator (not a splitter) is used to feed a separate cable to Satellite Input 1 and a separate cable to Satellite Input 2.
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