sg60, just a tip... i personally have quite a large library and often want music in multiple genre's as yourself, you can do as zxsix suggests, or you can also use keywords, i use keywords instead of genres, by doing so it allows me to select multiple genres. and above all be able to see them and i don't have to remember the genres i have in my library (over 40 different genres), where as using genres with semicolon, it's basically a guessing game, and with keywords your whole list will popup allowing you to see all available genres(keywords) that you use, and then you can just select each genre, hit enter and it will fill in keywords with all genres (it is quite a bit of work to change all your views, rules etc. but if you have a larger library and are very picky about having artists in multiple genres it may speed up your tagging process in the long run.)
the same goes for artist ( i have many songs where there are 2 Mainstream artists singing) example, ozzy and lita ford - close my eyes forever. well I wanted to be able to go to ozzy, and see ALL of his work, and when i went to Lita Ford i wanted to see ALL her work. So what i did was was made a new field called ArtistS instead of artist. and by doing so you can use the semi colon as well, and when you goto ozzy OR lita ford, close my eyes forever will then show up for each artistS.
just a warning before you dive in if you decide to go the route i took... if you use other media players, phones etc (outside of jriver), and use keywords and ArtistS instead of Genre and artist. your other devices tag info will be empty. because all other devices out there use genre and artist, NOT keywords and artistS... what i do to get around it is, every so often i will highlight all keywords/ArtistS and then do a move, copy, replace. and Copy all keywords to genres, and all ArtistS to artist. it's a little extra work, but I feel by doing so it meets my needs, and above all i have less babysitting with family when they are looking for a certain song... after all it is a 50/50 crap shoot to guess if close my eyes forever would be under ozzy or lita ford.
i'm personally hoping some day they will enable multiple Genres/Artist and tick boxes as they have with keywords and ArtistS.... it would make tagging so much smoother, QUICKER and robust.