Are you talking about two individual permanent magnet DC motors connected in series or a series wound universal motor?
As RODACOL was saying, if it's a universal motor simply reversing the supply won't help, it will stay turning in the same direction, you need to reverse either the field or armature.
If you're talking about two permanent magnet motors, then why are you connecting them in series? As you appliy more load to one the other will speed up? I don't see how that would be beneficial.
What are these motors driving?