You have many variants, the supertitles are very flexible:
1. To remove the supertitle from one forum and to put it to another. This is valid for every forum.
2. To change the place of every forum, too, up and down - at the bottom right side of the admin's forum setup page there are clickable arrows for that.
3. To put a separate supertitle of the hidden forum(s) - this is what many admins do, just to inform the visitors that thay aren't visible to the others.
4. You can even rename the forums - if you have your own very special order of the titles in mind - no problem.
Note, too, that 1, 2 and 3 are only forum's IDs - if you reorder the forums visually, nothing else, including the IDs, will be changed - you'll just see forum 1 under forum 3, for example.
I would suggest you the following simple thing:
a) firstly to name and to order the forums as you like mostly;
b) after that to put the sections supertitles regarding your need to separate the groups of forums.
You can after that make whatever new forums you want and put them wherever you find most convenient.