What you need is a PID control driven by your gyro.
For a simple approach I'd start with setting one motor speed to below maximum, e.g., control it directly with a joystick but cap the maximum at .8
That gives the other motor a little room to run faster or slower than the first motor while trying to keep things level.
Then use PID just on the second motor to keep the gyro level.
The right half of this example below.
Replace the "What we want the pot to read" with your target gyro reading.
Replace the "What the pot currently reads" input with the current gyro reading.
The target reading might be taken when the platform is down and level before it starts to rise.
You will need to adjust the PID coefficients by experimentation. Keep I & D zero and just play with P (start P at 1 for the first test and gradually increase it) until you get the platform to remain steady (but probably a little tilted) all the way up.
Then start playing with I to level the platform out (start with a small number like .1 and decrease it).
