@blackmetal couple of mis-conceptions. #1, Master/Backup pertain to RE (CPU) function within a [in your case] VC. The RE is the one that learns info, and then populates ever member of the VC with same info. This Master/Backup has nothing to do with how VC actually forwards traffic. This is unlike Primary/Standby for say a FW where only one node forwards traffic, while other sits and waits to take over in Primary fails. Master/Backup RE functions this way for learning, not for forwarding.
Your AE/LAG is 20G but for any single flow the max throughput will be 10G. Any single flow is always mapped (hashed) to same single 10G link. You could get max of 20G of total throughput (depending upon hash and traffic) but for any single flow, max is 10G.
HTH