I recently picked up a 2nd D700 body and (in a different transaction) grip. Beyond those two cameras, I use a D5100 as a lightweight/vacation camera in conjunction with a Tamron 17-50 2.8 and Sigma 30 1.4. My telezoom is a 70-300VR.
My backup body for the D700 is the D3s. I also have a D300 (as well as a D2x, D2h, D200). I find all the cameras to be very interchangable in how they feel in the hands (I have vertical grips on all the x00 ones) but the D300 and the D700 are so much alike I have to read the label to know which one I am holding.
My other camera is the D800. I really like both. Planning continue to use the D700 for sports and portraiture. I like the D800 for street shooting because of being able to crop in closer at post processing for what I was really intending to shoot. Also, use the D800 for macro and landscape. I used both recently for a marathon race with my 28-70 on the D800 and the 70-200 VR2 on the D700. Carrying both worked well.
Still using the D200 I had before I got the D700. If the light is such that I can keep the ISO under 400 it still makes good images. I use it for long telephoto when possible and love it for street shooting with a 16-85mm Dx lens.
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!
D600 is my back-up to my D700 now. When I want more reach I use the D7000 and my D90 as the ultimate always faithful backup for travel unless I want to go real light with my P7000 or my Fuji X10 and now my new toy Fuji X100 which is great for the street.
I have the D700 as my main camera and the D300, that I had previously, for use with subjects that benefit from the crop factor and long lenses. I like the combination very much. My 'fit in pocket' camera is a Panasonic Lumix LX-5 which is an excellent piece of kit. With these cameras and my selection of lenses I feel that I have most bases covered.
Like the OP, my other camera body is a D300s. With VERY similar controls and physical layout, it's easy to switch from one to the other.
I also have a D70, which I will eventually give to one of my kids if they show some sort of interest in photography (not so much yet). And for times when a DSLR is too big, I have a Canon G15 (don't tell the management).
I view the notion of keeping commonality of controls as a strength for Nikon, so I hope it continues with a D400 that has controls similar to the D300(s), and the same, substantial size, feel and build.
I'm using a D3100 with kit lens. I've taken the 3100 with me on vacations, in rough conditions and it has never let me down. At Seaworld, we sat in row three at the Orca show directly behind the pool and it took the brunt of several full splashes from the whales (I found out the hard way they are trained to splash you if you are standing up....say to get a photograph); granted I covered it somewhat but I was soaked head-to-toe and so was the 3100. But it had nary a leak and it worked fine when dried off. Got some great whale pictures, too!
I use my D700 interchangeably with several other bodies: D200, D300, D800E, D2X, D3, and D3s. The layout and controls are so similar that it is very easy to switch between them even on the fly without missing a beat. Good Luck and Enjoy your Nikons!
My second body is my trusty Coolpix E8800. It is light (12 oz), small, has a 10X zoom and gives me RAW. The only shortcoming is low light but we need a challenge every once in a while, don't we? Great travel camera.