Did you put them in series, or parallel?
I'll assume you connected their positive leads together and their negative leads together. That is connection in parrallel, and you get the same voltage with the capacity for twice the current.
To get 12V you need to connect them in series. Connect the positive of one to the negative of the other and the unused negative and positive leads will have 12V across them.
edit: Oh, in series you get the sum of the voltages (12V) and the max current that can be supplied is that of the LOWEST current of either charger.