Upload & Sell: Off
Started with D3/D700 combo, moved to just the D3 and then rented D3/D3s. Recently added the D800 to go along side the D3.
Out of the above cameras if I had to use one to shot a wedidng it would be the D3s, IQ is more than enough, robust and has dual card slots, file size not too big to slow things down.
As for the second body, I would use the D700 if I already had one, you can also put the grip on it and use the battery from the D3. If the D3s failed and I had to continue on with th D700 my only worry would be the lack of dual card slots. As the D3s and D700 use CF cards and are the same MP sensor you don't need to fork out on new memory cards.
I have not used a D800 in a wedding yet, but from shooting to date the IQ is amazing, I have thought about selling my D3 and getting another D800, but my concern is the workflow slowing to a crawl due to the size of the D800 files. 36 MP is overkill for the majority of a wedding, but the DR and IQ is so tempting.
The only other concern I would have with a D800 is it is a bit smaller grip wise than a D3/D3s and I have large hands.
As for processing files from different cameras, I use Lightroom and have calibration profiles made for each camera, so that makes the images from each camera very similar on import. It is really the effect of different lenses and lighting that causes problems when making a group of images have a consistent look.
I could wave a magic wand and go back on my Nikon purchases, I would get a D3s and D700, saves a lot of money.
So the cheap option is to keep the D700, but if you want the ultimate in IQ, keep the D800 but in case your D3s fails, make sure you have enough batteries and memory cards for that camera to shot a wedding and hope your computuer and hard drives can cope