Hi Ray, When you say "serious" shooting with the D700 does that mean printing large or for glossy publication? What do you do with the images from that camera? Are you using burst most to its limits now? If not printing large or needing the last little bit of DR and color depth the D600 excels at, the D700 probably does all you need to do. The D700 and D7000 make a very nice complimentary pair, which overlaps in the middle of each of their strengths. But for landscapes, printing large both the D7000 and D600 at base ISO produce more detailed, quieter files. The D7000 is a good stablemate for either of the FX bodies because it has the reach and extends the flexibility of your current lenses. I am using the D7000 and D800 and find they compliment each other very well for events, landscape, street and club/ballet shooting, but I am adding frame counts faster on the D800 more and more because I am finding it to be very versatile in a similar vein as how the D600 is wowing people over with sheer image quality. There are advantages and few downsides for either scenarios you ask about. Keeping what you have, getting two D600's and getting a D600 and keeping the D7000. For me, the latter option would fit my needs best but how about you?
Most would probably mention the build quality of the D700 as being a main reason for keeping it but I find that none of the recent models suffer in the ruggedness department. My D90 has taken more abuse that one would expect it to handle but its light weight means it does not need to be heavy or solid magnesium to survive falls and collisions. It still looks and performs great at the 84,000 shot point.
Another option to consider, a D800 which has enough resolution to allow DX cropping for the reach, plus unparalleled image quality in Fx. It could easily be your all-round camera for reach, landscape, low light, street, portraiture and action with all the lenses you have. Stan St Petersburg Russia