You have the right idea; the problem there is you're using what looks like a satellite diplexer. That device is intended to combine satellite and OTA TV signals on a single cable, so you can connect both a satellite dish and an OTA antenna to one end, and a satellite receiver and an OTA TV or tuner to the other.
Satellite diplexers work because satellite signals are much higher frequency and don't overlap OTA frequencies. But what you described earlier is that the satellite signals have already been received and remodulated onto (lower) cable TV frequencies, which do overlap OTA frequencies.
My idea with the UVSJ was to block out the higher satellite frequencies and the lower (VHF) OTA frequencies, so you could combine what's left without overlap and get a reasonable mix of both. But then I ran into the problem finding a UVSJ.
To answer your question, there are two types of combiners: one kind is just a reversed splitter and doesn't filter frequencies at all. Anything put into either input will come through to the output.
The other kind has a low-frequency and a high-frequency input. The low-frequency input filters out higher frequencies, and vice versa for the other input. This is the kind often referred to a diplexer, especially when it's designed to combine or split OTA and satellite signals.
Strictly speaking, a UVSJ is a diplexer, with the low-frequency input covering the OTA VHF band and the high-frequency input covering the OTA UHF band. But the term "diplexer" isn't commonly used for these, for some reason.