serial charging of two gel cell 6v..?

I have two 6 v 4.5 amp gell cell I wish to charge them both at the same time connected serially by using a 12v charger?
is this feasible? or do I need a 6v charger because they can't be charge by a 12v charger?
what say you, thanks,

If they are new, and identical, then you can charge them in series.