I haven't seen a rift I cannot close with 2 mages... But it is true that a warrior+healer can do a few things better, and being able to do Randoms more easily can be a big factor. I still think it's way easier to duo two mages for doing dungeons though and when you're in a guild it's not very hard to get the group together anyway.

One thing that is good with two necro/lock mages is that the abilities are ranged and don't require that you face the opponent. This is also very good for PvP.