Ya, think if it this way, if you have both wheels off the ground and you turn 1 wheel 1 revolution and come up with 2, then you hold that wheel in place and turn the other wheel 1 revolution it will be another 2. So no different than doubling your first count. If you're alone it's actually easier to jack up one wheel, mark the inside of the wheel with chalk, mark the driveshaft, then turn the driveshaft until the wheel does 1 rev. then double it.

I probably should say use a jack stand and chock the wheels on the ground, just incase.