Thanks. On my A3 you could hold the AUTO button and it could sync them both automatically. I was hoping the A3 would have something similar.Not quite what you mean by push and hold. You can use the MMI (not AMI) to set the synchronisation of the heating controls so that both sides are controlled by the driver's control - and I believe that it is only the temperature that is affected.
However, as soon as the passenger side control is operated, they return to independent operation again. I don't think there's another way of enabling/disabling the feature apart from the MMI route.