Assuming they get the same gas mileage:
133 1/3$133 \frac13$ miles.
Reasoning:
You drive both cars 33 1/3$33 \frac13$ miles. Then you transfer 33 1/3$33 \frac13$ miles worth of gas from one car to the other. That car now has enough gas to go another 100$100$ miles, and the other car has exactly 33 1/3$33 \frac13$ miles worth of gas left in the tank. That's just enough to get home.