I use Mp3Tag and can confirm that it cannot add more than one genre tag to a file.
It's a great program - works on more music file formats than you can shake a stick at, can batch rename all the tracks' filenames to [zero-padded track number] [track title] (without the brackets, and laid out however you want), and can web search for and add multiple cover images, but multiples of other tags are beyond it - and probably wouldn't be understood by any music players, being nonstandard.
If you search for my posts here, you'll find my method of cross-filing music by using "junction folders" - virtual copies of a single folder under different names. I use it to cross-file music under different people's names, but you could create a folder tree for each genre and create junction folders for groups that cross categories.
I don't use tag-based track management, because it will see the virtual copies as additional copies and play an album by playing each track as many times as there are "virtual copies" of it, before advancing to the next.
To avoid that result, browse using your player's ability to browse the network server's music folder tree, not with any tag-based aggregator. That way, your player will be looking at one virtual copy at a time and will play an album properly.
Also: remember that since there's really only one copy of the music files, if you edit or delete the contents of any of the alias folders, you're doing that everywhere, so use care! The only way that the various virtual copies can differ is in the name of the junction folder itself (the original source folder's name must stay unchanged or the links are broken).
Last edited by Philnick; 08-14-2014 at 02:04 PM.