this section of the forums is frightening to me with the levels of irrational, unreasoned rules-lawyering floating around. I'm sure my reply will be overly scrutinized.
Intent=
GW's intent is clear, the inclusion of the special rule "we are legion" and "watch this" indicate that the unit is supposed to 'feel' like one blue horror has cast a spell, and the other blue horror has also cast a spell, he may or may not have copied the first blue horrors spell in an attempt to show him up.
Lawyering= i believe in the description of the unit, they use the plural form when referring to the horrors but they state that the horrors ride atop a (singular) disc of tzeentch. sounds like 1 model consisting of a single disc of tzeentch with 2 blue horrors glued onto it.
if you are having a hard time figuring out if the cost of the unit includes 2 separate independent characters that can both shoot 2 gifts of chaos and can split off and join separate units, then I've given my reasons for thinking that isn't true.
If you like arguing semantics, the floor is open.