Predict the tankbuster. Better have Grit 5 seconds earlier on. GCD 1 and 2 of your aggro combo then provoke and GCD 3 of your aggro combo.

Provoke gives you only +1 enmity than the MT so you have to gain more aggro after this. Better is when your Co-Tank stops attacking for some seconds. Thats what I do when someone swaps with me.