Simon;
"hissing more than it should while idling" ? What do you make that statement based on?...SUs hiss because air is being sucked past that venturi restriction, so that's their absolutely natural mating call!...what you should see is both Dashpots being lifted similarly (but don't make yourself crazy trying to get the precisely equal!)...vacuum of engine running lifts them, so when engine is turned OFF, they should both drop completely down to bridge/venturi as vacuum goes away. If one does not fall completely, it either needs lube (dashpot oil) OR Metering Needle is bent/uncentered and making contact with Jet as Daspot falls, jamming it.
This must be resolved! I'd open both to inspect and lube before mucking around trying to synchronize!
I understand you're not an SU novice, but info here should still be of interest and use:
http://www.sw-em.com/SU%20Carb%20Service.htm
Good Hunting!