How do you sync the N1s on engines which use EPR as the primary reference?
The answer is you don't. Its quite common on the V2500 Airbus to see both engines showing the same EPR but different N1s. Its not really a problem unless one engine is much older than the other meaning it has to go round faster (and burn more fuel, higher EGT) to produce the same EPR as the other one. This is what produces the out of sync sound. The only solution, if you can be bothered, is to disengage the autothrust, sync the engines and just except the very slight power difference