K.C.
Is your daughter in therapies? How do her therapists respond to her tantrums? What do they recommend? They must have experience dealing with this sort of thing on a regular basis. If she's not in therapy, it sounds like she needs it ASAP. Without knowing her delays, it's hard to recommend potty training strategies. Is your daughter verbal? Could it be that she's throwing the tantrums because she is unable to communicate effectively? Two year olds often throw tantrums because they can't speak well enough or make their feelings/needs clear enough to other people. If your daughter doesn't communicate like her same-age peers, perhaps the tantrums are the result of her inability to express her wants and needs.
Work with her doctors and therapists closely. Whatever they recommend for her may work for your son too. Is he also delayed? Why do you think he has such violent tantrums?