Thanks for the detailed explanation of your antenna tests and the antennas and receivers you've used. I have always found RF reception to be extremely interesting (probably the main reason I got my ham license) and I enjoy trying to receive distant stations. I get KFTY-DT occasionally, but it doesn't lock up for long. I'm still trying to get a lock on a digital signal out of Sacramento/Stockton. I've got signals, but not enough to lock in a picture.
Using two 91XG's, as shown in that photo you posted, should definitely improve your signal levels, but I would think that if they were mounted at different locations and then combined together you might find that you get more stations more reliably. Whether or not the time and expense is worth it for you is the big question.
I have finally found an antenna arrangement here that solves most of my problems. As you've read from me in the past, I've never been able to find one antenna setting where I can get all of the digital stations transmitting from Sutro Tower... and I'm only 3/4 of a mile away from it. I've been able to get all of the stations, but have had to rotate the antenna to different directions for the various stations. I've tried it with both the CM4228 and the Antenna Craft D9000, an FM-VHF-UHF combination antenna, and I have the same problem with both.
A while back I saw a used CM4228 for sale for $20 and grabbed it. Using it and my original CM4228 tied together using a simple splitter used backwards as a combiner I now can get all of the Sutro channels without using the rotor! With the two antennas pointed in different directions but working together, I found a combination that works. Added to this is a Winegard 3113 for Ch12 using a Joiner and I can now watch all of the Sutro stations plus KNTV-DT without switching or rotating antennas!
Possibly positioning two 91XG's at different locations and combining them together will work for you like the two CM4228's working together solved my problems.
My next project is to try adding a CM4248 that I have to the mix, pointing it toward the East Bay hills so that I can get those stations too without messing with the switch and rotor. At some point I'll probably have too many antennas combined together and will end up with multipath hell, but I'll keep trying various combinations to see what works and what doesn't.
As for receivers, I haven't tried a 5th generation receiver yet. From what I've read there are now two receivers available using the 5th gen software, an LG and a computer card (can't remember the model numbers). I have three receivers here, the one built into my Sony, a Hi-Pix computer card, and the Dish 811. The Dish 811 is the best of the three. It seems to hold the weaker stations better and there are less video break ups with it than with the other two receivers. Your Dish 942 is probably equal to or better than the 811, so it should be one of the better ones, but it's still not 5th gen.
Keep us posted on your progress, Keenan.