It's actually a dps increase even in single target. It's all depends on your melee dmg and how many spirits are involved.
If we think about splitting our total damage at the end of a fight like as we did it by playing spirits, a preliminary formula could be:
1 spirit, % = (60+(100-melee)*0.6)-100
2 spirit, % = (40+(100-melee)*0.4*2)-100
That means, there is a dps increased for one spirit if melee damage is < 33% and for two spirit if < 25%
Actually, melee is 20% of total damage, so by splitting with 1 spirit and 2 spirits there is a 8% and 4% increasing dps.
For achieve the 120% damage as the ability says, our melee should be 0%, and that's almost impossible.