Have one key for assist this key is what yeh use to switch all yer chars target to current target

bound to some key
/cast target=Kuolemaa greater heal
/cast target=Kuolemab greater heal
/cast target=Kuolemac greater heal

or what ever heals yeh want to use
this way yeh dont have to worry about what yeh have targeted they still get the heal

then one or both warlock have /cast [target=Kuolematarget] fear yeh might want to add a stopcasting macro in front of it
this shouldnt change yer current target of the warlock

they wont change targets unless yeh hit yer assist button yeh can still spam spells

only other thing I can think of is use one of the warlocks as a head instead of the priest