I'll make a major test with DFP2 and thumbs deletion by tomorrow and give you my feedback. But if i'm correct either it deletes all files (based on DFP upload behavior) or you can't add a second DELETE FILE behavior as it is associated with the upload behavior.
If you got only 1 field in the db and using multiple thumbs with ofc [filename]_thumb or sth like that, then the delete behavior should - optimal- delete the original file [filename] and [filename]_thumb.
I think it is not working like this but i havent heavyly tested it yet but i will do it by tomorrow.