Without a schematic and an understanding of the circuit it would not be safe to bridge any of them.on mine psu there are 4 optocouplers so those that mean that i should bridge all of them or i should bridge the one that is responsable for the safty future
A problem with simply varying the output voltages (they are probably all arranged to vary in unison with a common preset adjustment) is that the voltage or current ratings of certain components will be exceeded, risking destruction/fire.