Dunno about heroic but in regular I wiped initially on them, if you have any sort of stuns it works great however. On the 2x pull I nuke one down fast (they should die before they can get a void zone off), then I hammer the second one with my pally while I nuke it down. Haven't had a void zone with this strat so far. And yeah, them dropping one on your dps pit will pretty much kill them in a few seconds.

For this and many other bosses I have shift-WASDQE bound as their movement equivs on my team and set to pass-through in keyclone, and simply do a shift-Q/E to strafe left or right the entire caster team, while my pally tank stands still. Has helped a lot on many bosses where quick movement is required.