Is this a problem of Mp3tag or of the player?
How do you define "most players"?
E.g. Windows Media Player uses ; to create multiple entries for ARTIST and COMPOSER. iTunes plainly ignores any splits.
So you would have to do some research to find the optimum standard. MP3tag will help you to keep it.
You should not mix the display and the internal format. To get track of tracks, the format as laid down in the standard is x/y. iTunes splits this data which may be nicer to look at (depending on taste) but still writes it back as what you see in MP3tag. The same applies to DISCNUMBER.
You can do that yourself in defining separate columns or fields that either show the left or the right bits of TRACK.
What you are looking at is the individual implementations in each player. If you do not like that, you should approach the developers of the various player. And you should not blame the messenger, MP3tag.