I tried to create a tag on SO that contained a plus character ("+"), but instead two seperate tags where used. How can I create a tag that contains "+"?
b-tree is the tag for both, b-tree and b+ tree. You do not need an extra tag for it. You explain in your question that it is a b+ tree, that's sufficient.
No-one clicking with "oh, a b-tree question, I can answer this!" in mind will be scared away with "Aaaah, b+! The evil master! Run, run!" after reading the question text.
There are already several questions which do exactly that (using
b-tree for b+ trees, not scaring their readers away).
You can't create tags with a plus in the middle, because .. well, just look:
is that questions tagged [b] and [tree]?
Incidentally, that's why when any tag contains a plus (we only allow it at the end) you must (and our system does) delimit tags in URLs with a space.