Yup if your going for max yield out of each tent more light will be needed. I don't know what your goals are I only have about 2.5 x 4.5 of my 5x5 in use with a 700 and a 400 and I could actually use a bit more on the one long side but I can produce more then enough to meet my needs with the watts I have.
If you want more lights check out the platinum lights or the topled reflector series, they have rectangle shapes that would fill your tent in better then 2 square lights would. It's pretty much impossible to evenly cover a square with 2 smaller squares.
If you don't need max yield on your first round you can always see how they do and how you like your coverage and then decide if you need more light for your next grow.
Sorry to hijack Doc, i just went through this exact same thing with my setup once I already bought my 700 and realized it wasn't big enough to cover as much as I wanted and then I realized there was no good way to only buy one more light and fill the whole thing. Luckily I don't need the whole thing
Are you going to start a journal?