The most important point is being able to get the first one off without your tank getting murdered. Options for doing this include Swiftcast, Presence of Mind, having a full health tank, or having a tank blow a few cooldowns.

Once the first one hits, everything is stunned until the second one hits, so landing the first means you are free to cast the second (though if your tank is at 50 hp, feel free to abort and throw out some healing).