Just to help illustrate the point that has already been made, here is a sample of my Battery Charge Log.
This shows a few charges for one of my 2s 8k 15C True-RC packs. You'll notice that when the voltage starts out lower, the battery accepts more energy in mAh and the charge time increases as well, which make sense. The "Side" row indicates which side of the DUO I charged it on.
Sync charge works for me, but I really never use it. Even charged separately, the cells all balance out to 4.2 so I really don't see the advantage personally of sync charging them.