If ur batteries are 7.2v and 2000mah".......
series would be 14.4v and 2000mah
parallel would be 7.2v and 4000mah
Pit doesn't matter how u do it, if u charge at 1c they both will have the same charge time.
The thing to see... If it's even possible... Every charger has it's limits... Watts. How many watts is the charger??
also the more cells u charge at a time the higher the chances of a cell been over charged because one other cell is a little low.
where if u charge them separately the packs as a whole will be closer to been balanced with each other as a whole