I did a lot of experimenting with multi line /castsequence.

/castsequence lb,lb,,
/castsequence ,,heal

The above will work, but ONLY if you don't spam the button, you have to wait for GCD to be up between LB casts (making this pretty useless). If you press before the GCD is over, the first /castsequence line fails (when you get "spell not ready" text) and the next one executes (before you ever get to the empty , in the first line).