You're out of luck, I'm afraid. The developers have locked the [Artist] field as a string (not a list) which means it can't contain multiple entries.
There is a workaround, but it's a bit painful. You can create a custom field (Tools > Options > Library/Folder > Manage Library Fields > Add New Field), call it "Artists" and set it as a list (semicolon delimited).
Once you've done that, you can select all of your music and do a move/copy fields (Right click > Library Tools > Move/Copy Fields) to
copy [Artist] to [Artists].
You can then Find & Replace (Right click > Library Tools > Find and Replace), select your new [Artists] field and replace " & " with ";"
The semicolon ( ; ) acts as a delimiter to separate items in the list. You can then use this Artists field in panes, views, searches etc. This does mean you're setting yourself up to maintain two artist fields, however...
Did I mention it was painful?
PS: Welcome to the forums