well, if you feel that you are doing reasonable in both subjects, i would continue with 12 units. You never know, it could turn out that another subject might backfire, instead of the ones you are thinking of dropping.
If i were you, i'd wait a little, continue doing the 12 units, and keep your options over. You'll know a little down the track if it is worth dropping any subjects. They say that 12 units is a bit stressing, but it is copable, and i regreat not picking proper 12 units (i did english ext and photography), in which i got automatically dropped to 11, and i dropped ext cause i knew that i wouldn't do that well in it.
All i'm saying is, stick with 12 units, it can help in the long run. Not dropping any units at all can mean that you could have a saving subject in the event 1 falls out, and that 1 that falls out doesn't always turn out to be the one you wanted to drop.