Devious - Like the 700's the Traxxas 775 will not bolt up due to the motor screw holes being in a different place. If you really want to do this then a very good option would be to get yourself a motor and ESC setup out of a Savage or E-Maxx. These setup come complete with a a pair of motors and the correct ESC to handle them and you can normally find them for under $100
Keep in mind that these motors are designed to run at a higher voltage and running them on 7.2 or 8.4V will make them slow.
A better option than running 2 separate batteries would be to invest in a 4S Lipo and aa low voltage alarm. This will provide the necessary power to make the motors spin at the correct RPM.
Those setup's have no problems pushing Savages and E-Maxx's up to about 25 mph so they will have more than enough power for the crawler.
I've used that same setup on other vehicles so i'm pretty familiar with it if you have any more questions.