Most preamps which have two sets of outputs have them jumpered together internally, so that they are both driven by the same internal circuit. Jumpering them together externally will not accomplish anything. Some preamps have separate driver (buffer) circuits for each of the two sets of outputs, in which case they definitely should NOT be jumpered together.
If you are referring to shorting plugs, which connect the center conductor of an rca connector to its outer (ground) terminal, those should never be used on outputs, only on inputs. Shorting an output stands a very good chance of damaging it, depending on its exact design. And I doubt that using shorting plugs on inputs will help a hum problem, although it is remotely conceivable that it could.
Regards,
-- Al
