User blog comment:Miskos3/Fairy Tail Chapter 379: Tartarus Arc, Part 3: Underworld King, Review/@comment-4449950-20140404073720/@comment-7418318-20140404104710

Combination attacks and Unison Raids are two different things.

Just look at Yajima's comments to see what I mean. He called the combo attack of Scorpio and Taurus a "combination attack", but he refered to Sting and Rogues White Shadow Dragon's Fang as a Unison Raid.

It's only a Unison Raid when the magic of the participants is combined before the spell is fired, while combination attacks merge already fired spells, like the Wendy+Scorpio example you mentioned.

There were only ever three confirmed Unison Raids in the manga: Lucy+Juvia vs. Vidaldus, Sting+Rogue vs. Natsu and Juvia+Gray vs. Lyon+Chelia (though I'm not completely sure about the last one), whilte there were plenty of combination attack throughout the series.

Plus two more in the anime. Here the one in the Daphne arc was done right, but the anime team messed up with the Levy+Cana+Lisanna one in my opinion, since that depsite Lisanna calling out "Tripple-Unison-Raid" really looked like any ordinary combination attack that even three unnamed Phantom Lord fodder mages were able to do.