are you using a gearbox so that the motors end up matching spin speeds? if so, it should work fine. if not, you might need to make a function to translate the value going to one set of motors into an appropriate value for the others... i.e. if your chips are on pwms 1 and 2, and the drills on 3 and 4:
Code:
pwm01 = leftvalue;
pwm02 = rightvalue;
pwm03 = drillTranslate(leftvalue);
pwm04 = drillTranslate(rightvalue);
the drillTranslate() function might just be a simple curve, or you might need to use a lookup table.