Assuming they get the same gas mileage:

>! 133 1/3 miles.

Reasoning:

>! You drive both cars 33 1/3 miles. Then you transfer 33 1/3 miles worth of gas from one car to the other. That car now has enough gas to go another 100 miles, and the other car has exactly 33 1/3 miles worth of gas left in the tank. That's just enough to get home.