Finally!!
I noticed that I was looking for a solution without explaining precisely the problem, so here it goes:
I use GENRE and GROUPING tags as multi-value fields, separating each value with semicolon. To me, GROUPING works as a keyword-mood-occasion tag, it means, contains values like soft, instrumental, classic, Japan (for music from that country), french (for music in that language), orchestral, soundtrack, funky, sunny, popular, party, acoustic, live, and things like that.
The thing is, I must be able to perform fast but advanced queries based in those values, just like Custom Search works but in a faster way (see my previous post
http://getmusicbee.com/forum/index.php?topic=14217.0), so I use Winamp in that case.
The way Multi Genre Splitter works causes that every value in GENRE are stored separatedly, instead of a string containing "value1; value2; value3...). This causes that Windows, Winamp, Tag&Rename and any other DJ software does not recognize any of the values but the first one. Personally, now that I understand how MB works, I prefer values stored separatedly but unfortunately I need compatibility with other software.
On the other hand, I just love the way MB treats values separated with semicolon as unique values, so using / instead of ; would be fine to get values stored as a string but also would make useless this feature.
So far, the problem exposed. Now the "solution" (it's not quite sofisticated but works for me, and hope for anyone else):
I managed to create a custom Advanced Search And Replace preset that takes GENRE and GROUPING and concatenates its values into PUBLISHER (I don't mind at all that tag) which is stored as a string and the Multiple Tag Splitter has nothing to do with it. Also set that rule to be automatically applied. That way I get my other-not smart software to read all values in the same tag (no, I don't mind that either, as long as they're all there). So, that's it!
Thanks a lot for reading/answering to this post.