Multiple battery charging

i have 2 batteries in series. can i leave the neg on both batteries and seperate the pos and charge both at same time with one charger?
if not can i leave the neg conected and seperate pos and charge each batery,not at same time? trying to save a little time and work.
it seems like it would be ok to leave the both neg post conected because im breaking the circut by seperating the pos.