I had a D7000 for almost a year before I got my D700. Those two bodies made a pretty good combo but their controls were somewhat different. For me, I needed the similarity of controls to help me with shooting consistently. The D7000 got little use after I started using the D700!
My wife wanted the D7000 for its low-light performance (much better than her old D5000). So, I let her have it and bought a D800. The D700 and D800 make a great pair. The controls and settings are very similar, with the D800 just having more capability/features. My brain doesn't have to think about which camera I am using so I can concentrate on composing and shooting.
I think that I would miss the D700 if I sold it. Also, I have an occasional need to take a fast series of action shots and it is the better of the two for that when I use the MB-D10. So, for now I am keeping - and using - both of these fantastic cameras!