I have just recently started using Mp3tag, and found it to be very useful. However I can't seem to figure out how to append the artist tag to the title tag and enclose it within a pair of brackets. I can convert tag to filename, filename to tag, and filename to filename, but there doesn't seem to be support for tag to tag operation. Can anyone please help???? 
Look at this.
In your case, Action would be something like this:
Action Type: Format value
Field: TITLE
Formatstring: %title% (%artist%)
Regards
nickless