Negitives:
1. Less dmg = more hits = more times it will try to hit a mob down.
2. If it is at half dmg it means both mercs have to be hitting at the same time to do the same as what 1 Merc can do, if there is at any point where one of them is not attacking, not worth it. However same could be said the big Merc. Depends on your luring skills, except 2 Mercs is simply too annoying for it.
2. 2 Mercs hitting 1 target = ofc higher chance to hit down. from combo / jump atk.
3. If you split up a low hitting low hp Merc, it will simply die right away and be unable to kills mobs by it's self on higher lv maps.
Positives (kinda?):
1. Splitting them up to kill multiple mobs would be annoying as fk, and only benefits you if your doing a map like lv 60. (See Negative #3)
2. Dunno.