User blog comment:B214/Question and Answers 2/@comment-117.196.181.215-20140224174554/@comment-24501441-20140225014052

Personally I don't see an issue with two future heirs marrying eachother, but one would probably have to abdicate their future title for it to work. Maou's already abdicate their title to become Maou, so I wouldn't see a problem if one the future heirs did the same thing. They would still be spouse of a clan head so it's not like they would be losing that much prestige. But since marriages in the underworld seem to be set up by the parents (and I'm sure each clan would want their child to remain heir and therefore not agree to conditions) I don't really see this case happening that often. More often than not it's more like the Rias/Riser situation in which one was a clan head and the other wasn't.

For (completely hypothetical) example:  If Sona and Sairaorg were to become married, I could see Sairaorg abdicating (he has a younger brother) his future as Great King, especially since his goal is to become Maou, in which case he couldn't be Great King anyway.