... Is it intended functionality that this can't be cast unless you target both a creature with flying and one without?

Well, I didn't intend for the player to be able to only hit one target, so I guess.

Seems overcosted then. Interesting flavour though - swearing blue enough to cause birds to plummet onto their heads?

Yeah, this is Branching Bolt except forcing you to have both targets, but costing more rather than less.

­Clan Defiance is another similar card but Branching is spot on. The cost and the awkward targeting requirements remind me of Decimate.