Hey Marc, thanks for the input.
I thought you worked with the A7 (or A7R)..which doesn't have a dual card slot either.
I know a lot of people who work with a FF and an APSC as their backup and for the longer range with certain lenses.
My biggest fear would be high ISO...where my A7(s) shine.
I primarily shoot weddings with a Sony A99 (dual card), and before that with Nikons and Canons with dual card capture. I also use a Leica S, which also shoots to two cards.
Any single slot camera is used only for images I can afford to lose (random candids, some detail shots, etc.).
Personally, I don't consider my A7R as a pro wedding camera (too loud, to slow, finicky control ergonomics, VF lag). Okay for some stuff, but I could just as easily leave it at home. Same for a Leica M, which I also occasionally use at a wedding.
Use of FF and APSc as back-up seems reasonable ... but I don't need reach at a wedding ... long lenses tend to compress the perspective too much IMO. My ZA135/1.8 is enough for 98% of any long lens need ... for the other 2% I just crop the full frame to APSc (which you can do in camera)... and the effect is the same as a 200mm.
I avoid high ISO in most cases since available light at most weddings is pretty poor in quality.
_ Marc