set them equally, use a DMM to verify both channels are equal. remember that when bridging we are giving the speaker/sub a signal on both wires, just one is out of phase by 180 deg. you want the amplitude to be equal in both signals and to do that you need to adjust both gain knobs.
if the amp has a switch specifically for a bridge mode - then i can see it only needing one knob - but that is a specific design and would be explained in the manual.