Now, which is better? *Should I just say thank you or say that I had
done this before and it didn't work. *I think the signal was too weak
from one or both sources after having gone through the mixer/splitter
reversed.

By your initial post I guess you are using an A-B switch because both
boxes output to the same channel. Isn't it the fact? If you already
set them to different output channel and they still cause interference
it is either because they output a lot of harmonics (low quality
modulators) or the TV tuner cannot filter an adjacent channel
properly. In this case the solution would be to plug an external VHF
III or UHF modulator to the one causing most interference.