Unison Raid

Unison Raid: During the Tower of Heaven arc, Lucy and Juvia gain the rare ability of Unison Raid. This gives two users the ability to share increased magic power for a short amount of time by fusing their magical powers. It seems that for Unison Raid to be possible, you have to use similar types of magic as both Juvia and Lucy used water magic at that time (Juvia = Water Magic, Lucy = Aquarius)