Yeah, two bodies is surely the way to go if you're going after wildlife or shooting sports, preferably including at least one that'll AF at f/8, so you can put a TC on that 400mm.
Despite how it slows down my 5D3, I can't bring myself to take off the 2.0x TC-III. One day I'll get the 1D X so that I don't have to struggle with AF so much, but the struggle is worth it.
Shooting out of my car-blind:
More Doe Eyes by
dcstep, on Flickr