Are you using the same address? Or computer? When Mahayana joined PT we had the same problem. All it took for him to post was for me to log out and him to log in.

This is done by clicking on any "logged" on any post, which tales you to a screen that says "you don't have permission..." at the top, and a place to logout at the bottom.

On the next page we click "user cp" at the top. This takes you to the same kind of "you don't have permission page", with a place for him to login at the bottom.

We do this each time one of us wants to take a turn posting. Hope this helps!