Originally Posted by fluffs

If you have it in the opposite order, single-target skills can finish off one of the enemies, halving the damage your aoe spells do.

It just doesn't have any sense. If girl with single target skill can finish enemy by herself then she won't deal all dmg she's able to if she attack enemy weakened by girl with aoe skill. Either way one of them won't be using her full potential. However if girl with single target skill would be after girl with aoe skill then it might happen she'll use her skill on almost dead enemy while other enemy would have more hp.