the two line cast sequence should not work simply because wow views an empty cast sequence as a successful non-cast and thus will not go to the next line. Or at least it shouldn't work.
one I am not sure about is having a secondary target "might"

something like /castsequnce [target=target] [target=maintank] LB, LB, LB, Heal, LB, LB, LB, Heal

as when it hit's the lightening bolts it goes with the first target as long as it is a legit target for the spell. Once it hits the heal, the (presumably) harm target is no longer a viable target so it should go with the secondary target.
I have not tested this, but it is a theory. :P