They found a way to do it without turning any participant into a clown with the Battle for Dazar'alor "NPC tells you what happened" format if they really want both factions to experience it. But, this being Blizzard, they refused to use this method ever again so instead we've Genn inexplicably helping bring the Forsaken plague by proxy. It's absolutely retarded, as all these crossovers are, and they should be stopped.