A diplexer is basically a splitter/combiner designed specifically to merge/split low-frequency antenna and high-frequency satellite onto one cable... it's not really what you'd want to use to combine two antenna signals. There are higher quality normal splitters that usually do a better job of combining than a normal splitter (look to Channel Master).

Filtering out frequencies in the UHF band is a difficult and costly idea - the Join-Tennas are only designed to do ranges of UHF channels (14-29, 30-49, 50-69), not pick and choose specific channels from various antennas. Basically, combining two full-frequency UHF antennas aimed in different directions is not a good idea. The equipment to do this sort of thing exists, but it's only sold on the heavy commercial level (ie. cable companies).

Go with a switch or a rotor.