Wondering if I can add a 2nd antenna (add a UHF), and merge the two signals on the roof?
They only offer about 0.5 db loss, which in most cases won't change a thing.
If you put UHF and VHF on the same mast try and separate them by 4 ft if possible.
Depending on hills and stuff if you can see the mountain transmitters you would not need a big VHF antenna.
This maybe overkill, but it will work if you see a few bars now from VHF on your UHF. It's inexpensive also.
AntennaCraft Y5-7-13 Highband-Broadband VHF HD Yagi for Channels 7-13 (Y5-7-13) | Y5-7-13 [AntennaCraft]