Well, I usually have some poorly composed, some blurry, some wrongly exposed shots, and if it is sports or street photography, it can be a lot of junk.
As to the synchronized comparison, that’s a good point. For me, using 2 instances of FRV is comfortable, but I can see what you mean. You should try contacting the FRV developers through their site to let them know you need this feature, they’ve been pretty good about reacting to customer demands.