I know, but if you ask an advisor directly, they mention it, like the "one degree only" clause, which is not mentioned on the web site information either.
The modules allowed (or proscribed) for each qualification are mentioned on the qualification descriptions, the OU is currently updating its web site. If you want to start now, ring them, if you want to start in the autumn, wait until end-March when they will put up the new details.
I'm not studying humanities, so I can't help with humanities degrees, I would suggest ringing the OU rather than picking up hearsay.