I had a similar situation. Had a D700 and a D7000. Ultimately sold the D7000 as the DX mode on the D800 took care of the reach issue. The DX mode at almost 16mp was my determining factor and at one point I was going to sell both the D700 and the D7000.
Still have the D700 and the D800 and love having 2 FX cameras. I use both cameras frequently and have not regretted selling the D7000 and keeping the D700.
Absolutely love the D800, but the D709 is still an excellent piece of equipment.