When we got the presentation from Specialized in South Africa they said that draining the RE first was not the optimal way of using the battery and there would be a significant drop off in power nearing the end of the RE's battery. Hence, don't do it.
Even on a standard Levo at 3-4 bars the power output is less as the Voltage drops from the battery.
The idea of using RE first is that if you have more than one, to use RE first and loop back and swap out for another. It's not recommended if your going to ride a normal loop with a RE to use that first, then switch to Main Battery. Go for dual discharge. You do need the Y lead to charge both.
Always go for dual discharge in MC if you have a single Range Extender plugged in.
Unless you're a machine I can't see a need to use 2 Range Extender's and the Main Battery, your're talking about a 50-60 mile loop and just swap the RE out when it drops if that's your bag. Rob and I rode and his discharge rate was almost identical to mine on the FF Levo.
@TomH Were both batteries fully charged? The software is smart and from what they said at the presentation it will balance them, but I would assume it needs them to be somewhere near equal to their full capacity or it might not handle the discharge of each efficiently. From what I remember of the talk they said with more than one Range Extender the logic should be in MC to use RE, RE, Main battery. Otherwise, with a RE and Battery, it was discharge both at the same time. That would imply, and I'm speculating, they all need to be at full capacity.