Keep in mind that altough you can DX crop the D800 down to a near D7000 equivalent (in camera or in post), there will be a difference in how the subject appears in the viewfinder. With the D800, it will only cover the DX portion of the viewfinder. Some people prefer the larger view of a DX body for long lens wildlife.
If your getting the D700 to be the high fps part of a D700/D800 duo you should probably think of adding the MB-D10 grip. The fps of the D700 alone is 5 fps vs 4 for the D800. Add the grip to the D700 to go up to 8 fps. You might even consider a used D3. Same sensor as the D700 with 9 fps.