Do you actually have a use for a third camera body? You have two great cameras and each can act as a backup for the other (even though they have somewhat different specifications). The fact that new, warranted D700's may not be available in the future also applies to all other camera models that have been out for a while. Do you feel a need to get a D300s, D90, D5100, or some other model? These cameras also will be unavailable (as new with warranty) sometime in the not-to-distant future.
Since you are asking this question I suspect that you have not settled in your own mind why you should purchase another D700. If you want it, and you can afford it, then buy it and enjoy it. If you or somebody else would have to give up something to be able to purchase the new camera, then you must make a decision about which is more important: The camera, or the "something else."