... if I get a successful signal strength from those 2 channels on a separate mast, could that be combined with another mast on the other side of the roof without problems?
"could be combined" ? sure
"without problems" ? . . . welllllllll
I don't build community antenna systems, but I do think you'll probably need some semi-professional equipment to get the job done, given your non-optimal circumstances.
First, ya gotta -get- the signals, and they've got to be clean.
You haven't demonstrated that, yet.
Some of your hard-to-get signals are 25db down from the rest.
If you were to get bandpass filters for just those weak stations, you'd also have to research whether they will change frequency come Feb 2009.
I have an ex-TVstation engineer buddy who pre-amps and pre-filters the off-the-air stations at his house, then recombines them with commercial modulators fed by a bank of Dish receivers , throughout his house.
Next time I chat with him, I'll try to get refreshed on exactly how he did that.
In the mean time, I think you need to find an expert in the field.
But once you have a workable plan, it's possible to get much of the equipment surplus, so cost need not be astronomical.