I can see nothing wrong with the proposal. Would merge+synonymize.
updating post history, 247 rows affected
updating posts, 204 rows affected
updating PostTags associations, 159 rows affected
removing/renaming old tag, 0 rows affected
updating count for master tag, 1 rows affected
tag remapping of [multiple-monitors] and [dual-monitor] complete!
remapping 1 synonyms
6 favorite and ignored tags remapped!
Tag Synonym dual-monitor -> multiple-monitors was approved!
Broadcasting tag-engine reload
updating post history, 28 rows affected
updating posts, 23 rows affected
updating PostTags associations, 16 rows affected
removing/renaming old tag, 0 rows affected
updating count for master tag, 1 rows affected
tag remapping of [multiple-monitors] and [triple-monitor] complete!
remapping 0 synonyms
0 favorite and ignored tags remapped!
Tag Synonym triple-monitor -> multiple-monitors was approved!
Broadcasting tag-engine reload