Just be careful… ID3v2.4 is not commonly supported (ID3v2.3 is the de facto standard, and all signs point that it will continue to be)… and folks will often run into compatibility problems. (Not to mention most applications have no concept for multiple names or genres in their UI.)
Personally, for Genres… I’d just use a single entry with slashes or semicolons between the Genres like Ron does. That way it’s compatible with everything… and, if Ron adds his data massaging feature, the multiple genres could officially be parsed into discrete entries for his “next generation” database using “/” (or whatever you pick) as a delimeter.
(Trivia: The ID3v2.3 informal standard defines “/” as the official delimiter for multiple names in artist related fields… Artist, Composer, Conductor, etc. But everyone ignores this, except a handful of applications.)
Better solution? Use formats that have always supported multiple names/genres natively. FLAC, OGG, WMA, etc. Of course, applications still need to support it, too. But at least you get one thing out of the way… 🙄