Why not use a UHF/VHF combiner network? They used to be sold in
electronics stores like Radio Shack, and come in a couple of versions
including all 300 Ohm, and 300 Ohm antennas into a 75 Ohm set.

You can´t do that. Its too easy! The solutions HAS to be complicated. That
is the first rule of homebrew. How else do you intend to impress
(uneducated) friends?