The issue will be is sustaining both at respectable numbers.
When DiabloIII was released, WoW's subs tanked hard.
When Titan is released expect WoW's subs to nosedive, as it's the next IP so many WoWers been waiting upon (as Cata and MoP are truly filler expansions until then). They will stay in the game for a good bit too, as it'll be so new.
When they ate through the content, then they'll return back to WoW, but the player base will be ping ponging between the games, and WoW's subs are going to look like a roller coaster in return.
I doubt the subs will remain stable, it's very difficult to play end-game in 2 games as once. It takes a lot of micromanagement and scheduling, as not all will play both games at once.