Own both 5DM3 and 7D2 ... w/ 16/35, 24/105, 70/200, 100/400 -- that combination gives a range of 16 >> 640 (inc crop factor, if you believe in it?) and with an 1.4 extender, anything farther than that I'd not even try to shoot ...
I usually keep the 100/400 v2 on the 7D2, and use the 5DM3 as more the 'walk-around' lens when I'm in the field ... I shoot mainly wildlife and outdoor sports (some indoor basketball) ... I find that combination allows me the fastest access to the lens I need because the 7D2 stands ready for the sudden/surprise wildlife shot you don't want to miss, and the 5DM3 with a couple lenses at the ready is much more versatile when I have a little more time to choose.
I like the fact that both have almost identical controls ... This camera combination is difficult to beat when using a two-camera set-up - at least in primarily outdoor situations. I rarely shoot indoors, so cannot comment it.