For me, money spent on lenses is usually better well spent than on a camera. But of course this is NOT a rule!Another 5D3 will just add to the muscle memory your probably built already. To me it's seemless to work with them as a pair.
All I need is BigValueInc sale, 5D III for $2500
Yes, that was a very good price!
Unless you have a critical need for a second body now, I'd hold off and wait for this winter. Hopefully, prices will drop to that level again, or at least close to it. You could always get the 400 or 200-400 first. With those big lenses, no one will realize a second body dangling off the end of it!
The are professionals who cannot afford to fail, amateurs visiting once in a lifetime places, specific requirements to be met by any kind of photographer, etc.