I havn't done dual in a long time, but I'll throw this out there..

I had two Lehner Basic 5300's on 12 cells each in my hyper7 buggy..
I later went to a single Hacker B50 8s on 16 cells, and the power was about the same, cost less, and weighed less..
I know those aren't very good examples, as both were out of the rpm spec, but from my experience, you're better off with one motor, than two less powerfull motors.