Just have it learned by 2 people. That's what I've done. Both my mage and my priest have set and return, and I use them frequently to jump back and forth to Arnika. It's not as hard as it sounds, since both spellbooks are easily available to find, buy, or steal.
As to your other question, yes, they must learn both. One spellcaster cannot "return" to a portal another has "set". So, having a character just learn "set portal" is pretty much useless... unless you like to waste spell points, that is!
