It's not a perfect solution, but you could do this with a metacourse. Create a new course and set "Is this course a metacourse" to Yes. Make your two existing courses children of the metacourse. Put the forum in the metacourse. For convenience, put a link to the forum in each course. Hopefully you won't be using grades in this forum (that would complicate things, since the grades go in the metacourse gradebook).
Similar to above you could use the "Social Forum" format for the metacourse, which would perhaps help navigation and prevent users from coming back out to an empty course page. The link would then be to the course. You may want to change the language slightly to reflect your purpose.
You should be able to use HTML block (or similar) to link back to the main course page.
Yours
James
You should be able to use HTML block (or similar) to link back to the main course page.
Yours
James