Back in the old days of KX-175B radios, having two radios made a real difference. When flip-flop radios came along, the difference was less. I used to be big in the two radio camp, and my RV-9A came with two radios. However, as part of a panel clean up, I ditched the second radio -- in part because the first radio had a monitor feature, meaning that when there was no traffic on the "main" frequency, the radio would automatically monitor (listen to) the secondary feature. Frankly, this works for me, even IFR out of an airport within Class C airspace.
Backup comm radio? Not concerned. If I do lost comm IFR, there are procedures for that, and radio failures these days seem rare -- I'm not sure I've heard of one in the last few decades.
There have been RV-8 panels built with two, 10" screens -- but figure out what information you'll want to display at once, and how many screens you'l need. With four years of G3X Touch under my belt, I find that I *never* use a full screen display for anything -- the sides of the display have no information of value. If I were putting in G3X Touch in an RV-8, I'd consider one 10" + GTN750, or one each 10" and 7".
I think you're asking the right questions. I've found that many people have strong opinions on how good their choices are when what they really mean is that they have come to like the process of choosing and installing. This goes for many other things, too, like which is the best airplane (think Bonanzas).
If you ever make it to Savannah, I'll give you a ride and let you fuss with the screens in flight and make your best decision.