User blog comment:Miskos3/Fairy Tail Chapter 379: Tartarus Arc, Part 3: Underworld King, Review/@comment-4449950-20140404073720/@comment-7418318-20140404101524

Unison Raid is about linking magic powers, not combining spells.

Just look at the one Lucy and Juvia made during the Tower of Heaven fight against Vidaldus. There Aquarius actions were already pretty much over when Lucy and Juvia joined their powers, meaning that the control over the water most likely came solely from Juvia, while Lucy "only" provided extra magic power for Juvia to work with, in a way of speaking.

So Erza could just provide magic, while Jellal provided the spells.