AvBryce hit it right on the head! I recently bought a sw tank that was a 10 gallon. Granted it was only a 1 1/2 hr drive, so no power head/heater was needed, but i did the exact same thing. Rocks/corals in one bucket, fish/livestock in the other, left the sand in the tank. Make sure you keep a heater/air pump in the car, and perhaps a low flow powerhead. This will be fine. Just make sure the 2 buckets are near the same temps when putting them back into the tank. Other than that, should be a pretty safe and easy move.
HTH
-Stewie