Well, I wouldn't really recommend stating it the first way:
Putting 2 in series will still be charging at 600W but double voltage, right?
because while strictly true it is not the best description if you're trying to discuss total power (Since AFAICT Doctorbass's statement you were replying to was talking aobut total power).
Instead, you are charging at 1200W, at whatevef voltage and current the total of each is (so twice the voltage of one, but the same current as either). It all means the same thing, but one is clearer (to me, anyway).