Personally, I would treat hybrid lists as Other Realm for anyone who didn't have both realms. Because otherwise it lets casters get spells that are effectively not from their own realm(s).
I probably would not allow them to start with the list at all, unless there is a good explanation that is central to the character concept (unlikely). If they want to learn it from another PC at the high cost, ok. It would be difficult to get taught in game, any teacher would quickly decide the character "does not have the aptitude" to learn it, and is unlikely to want to waste their time on this difficult student.
I do believe in fairly strong separation between realms. You might rule differently if you see all magic as the same and the realms just as three styles of casting.