Ah, then a single wire from the ground on the arduino battery to the ground on the motor battery (and put the servo ground wire on the same ground as the motor) will do the trick. The schematic will look the same as the connectivity is identical, its just arrangement in the real world that is important. Depending how much current the drive motor takes and how low a voltage it will take while still giving you enough torque to drive your chassis, you may be happier with a pack of AA batteries or
NIMH or LIPO (fire risk though, I usually avoid them) rechargable batteries. The 9 volt batteries are low amp hours and the motor will probably drain them fairly quickly. I'd try a four pack (6V) of AAs and see how the motor runs with them
Peter