The easiest way to scale to prevent set values outside of the -1 to 1 range would be to divide everything by 2. Unfortunately, this would reduce your maximum forward speed (and the others as well) by 50%.
It would also be cleaner to get the joystick numbers once rather than call the get() methods repeatedly.
A fairly general way to combine the numbers which allows full speed but prevents bounds errors on the set() methods could be:
Code:
fwd = stick1.getY();
rot = stick2.getX();
scale = Math.max(1.0, Math.abs(fwd) + Math.abs(rot);
frontRight.set((fwd - rot) / scale);
backRight.set((fwd - rot) / scale);
frontLeft.set((fwd + rot) / scale);
backLeft.set((fwd + rot) / scale);