Far as my group's been doing (run with friends here) the MT will tank the one that spawns the closest and the OT will tank the one that comes from further away. But that is just during the circle-strafe phase not during the eye of the storm phase. As that happens the DPS focuses on whichever has the green link, regardless of which tank got it.

During the eye of the storm phase I can't quite remember how we do it, I believe that the OT gets the one with the red link but I can't be sure.