See the FAQs /t/967/1
Create an action of the type "Format value" for TITLE
Format string: %title% (%artist%)
and even though this is possible - I would not do it. As the brackets are no valid separator if you ever want to get rid of the artist in the title again. Too many titles also contain a string in brackets that is not the artist.
I do no know any player that cannot at least display artist and title so that merging the 2 fields is not necessary.
If you want to get a filename with both pieces of data, use the Convert>Tag-Filename function
Format string: %title% (%artist%)