Hi,
it's a common knowledge, you can use (all) extenders with the 700-200 f2.8.
Certainly if it's already working with your 70-300 lens...
The glass of the Tamron 2x extender is sitting quite deep, so it won't touch the rear glas of the lens.
A Nikon extender looks quite different and also fits the 700-200 f2.8 lens.
Ad B