Separate them totally! I've already accidentally clicked the post button twice!
Luckily the posts were basically final, so I didn't look stupid or post trash on a thread, but this NEEDS to fixed, and probably isn't too hard to change.
More distinction between posting a reply and sending a private reply.
Part of the problem now is that they both look very similar. I think you need to move the post button back towards the top of the "Reply" screen, between the "To:" box and the text box, that would solve both problems.
The buttons must be below the fields.