Fantasy Book from the 80's

Hope you can help me remember this book! I read it in either the late 80's or early 90's. From what I remember, it was about two identical teenage boys. One was a typical teenage boy in 'our' world, who went to school and had trouble with bullies and girls. I think he might have been a bit weird as well (like maybe spoke a 'made-up' language). The other was a prince in a fantasy world with magic. I think the prince was in danger so the court magician (or similar) arranged for him to switch places with the boy in our world (they looked identical). Both boys flourished in their new worlds. The prince fought the bullies, got the girl etc. The other boy settled in and I think outsmarted the prince's enemies? I think in the end they decided to stay in their new worlds. There was also a big reveal like maybe they had actually been switched at birth so now they were home or they were really twins? Something like that.

\*EDIT\* I live in Australia - not sure if that is relevant but I think back then some books were only published locally, so it may never have made it to worldwide publication?