Ahh that makes sense then. I assumed it was a nuke you cast on the mob that has a twincast effect that hits the targets target and the next heal to hit that player hits twice. Its strange though that it when I was casting it on raid targets I wouldnt get the same message casting it on general UF mobs (or maybe I was working against the raid and I was nuking our own peeps whilst healing the raid mob...oops) unless its somehow getting lost in my battle spam window or that spell is flagged as not usable on NPC's/Mobs.
Will try hit the one on the top of agro list with it next and see how it goes. Will report back with my findings
[Update] Tested it out last night casting it on the tank and seemed to work well, saw the healing twincast message etc etc so looking good. Wont have much application in raids (due to clerics being main tank healers) but groupage should work well