I would not combine the antennas that way. I would keep the (UHF-VHF combined) downleads completely separate until I reached an A/B switch or some other isolating device. Combining antennas on the same band is a whole other technical issue, which I would avoid. You can combine the VHF and UHF signals on the same cable, since they are different frequencies and don't interfere. Your tuner filters out the frequencies that it wants and rejects the rest. A second tuner or an A/B switch would give you the kind of isolation you'll need to have multiple sources (antennae) on the same frequency.
http://www.solidsignal.com/pview.asp?p=tv-0151
http://www.silicondust.com/products/...dhomerun-dual/
I would use a new mast mounted preamp to combine the new antennas. The RCA preamp
http://www.amazon.com/RCA-TVPRAMP1R-.../dp/B003P92D9Y is recommended often - look at old posts for discussion. I have the Antennacraft 10G221
http://www.antennacraft.net/Antennas...mplifiers.html (not in service yet) since it was recommended to me by the experts here. Or you could move the CM7777 to the new mast and use the RCA on your existing installation.
Your other option is a UVSJ
http://www.solidsignal.com/pview.asp?p=uvsj but it has a 0.5 dB loss.
Realize that these masts need to be far enough apart so that the same-frequency antennas are isolated electrically. I would guess that's a minimum of about 6', though 12' would be better.
http://www.kyes.com/antenna/stacking.html