In your place, assuming you've got a strong desire to go FF, I'd sell your 7D and lenses, get the 5DIII with 24-105mm kit lens and then add a fast prime.
The 5DII AF really is that mediocre. The center point is decent, but then you have to focus/recompose, which with a fast prime means backfocus. Tracking is not nearly as good as the 7D. The other issue is the very poor spread of AF points on the 5DII - they're clustered around the spot metering circle, none even close to the rule-of-thirds intersections. It has the worst spread of points (relative to the frame) of any current Canon body.
No real issues changing from 5DII to 7D - the ergonomics are pretty similar (except for recording movies, which I don't).
I don't bring two dSLRs anywhere, I pick one. However, I also have a PowerShot S100 (and an S95 before that, now in my wife's purse) that I take on trips as a backup, and also because in some places a dSLR just isn't practical.
Personally, I'm not planning on going with just the 5DIII, in fact, I'm not getting a 5DIII at all - but that's because I'm getting a 1D X. I'll be selling the 5DII after that, keeping the 7D. I do often shoot birds/wildlife, and that's what I use the 7D for, as well as for shooting indoor sports (I prefer noisy and in focus to cleaner but OOF). The 1D X will be used for that going forward (along with all the other stuff I currently shoot with the 5DII), leaving the 7D only for birds/wildlife.