Filenames have nothing to do with the tag fields and can show more or less any information as long as the filename is unique.
Redundant information in tag fields should be avoided.
Structured information in separate tag fields is superior to unstructured information in a single tag field.
So, no: don't merge TITLE and ARTIST.
If you want to get basic information into the filename, think about how you search for a particular file:
do you use the artist first and then the title? Do you want to see tracks from an album grouped together?
I would use for albums from an artist: %albumartist% _ %album% _ $num(%track%,3) _ %title%
and for compilations: %album% _ $num(%track%,3) _ %artist% _ %title%
This is a personal choice, everyone has their own preference for what they want to see. Just keep in mind that many players, especially mobile devices and car players, limit what you can see when playing. Most do not even have some kind of scrolling function to see longer tags. So whatever you choose to do, make sure the most relevant information (I.e. the actual Title in this example) is the lead bit of information.