The biggest thing is timing your provoke and positioning the add.

Honestly, it's probably easier in the first add phase to just immediately jump on the close add with Rage of Halone (Combo) to hold it in place, and then use provoke on the other add when it comes in. A good main tank will also toss a provoke and ranged attack on whatever add is not being tanked by the offtank.

Definitely hold on to provoke until the red add is within melee range, because otherwise it will be completely moot.

If any of the healers use medica or pre-buff others with regen your chances of picking up the red add if it's coming from the outside go down dramatically, unless you time provoke properly.