Every autistic person is different... Depending what their triggers are and how they meltdown.
Work with what you have got.
We are an autistic family that tends towards being a bit shouty during meltdowns. Three generations of shouty meltdowns doesn't help the latest generation.
Teach recognising triggers and solutions. Try prevention rather than cure because by that point it is a bit late to bring it back fully. If it has got out of hand teach where to go to calm down and give it a lot of time to clear the adrenaline from the system.
Regulate with food, drink, ambient temperature, quiet space,not too many demands ( which is tricky if one of them is making demands) praise the behaviour you want. Name emotions. Teach them how to say what they want politely.remember that their communication style is different. We talk in imperatives so that the message isn't lost in a million words. (Name, shoes on now) build in relaxation daily so that the baseline level of agitation is brought down.
Having said that out of the two only one is shouty. The other meltdowns differently. The shouty meltdowns can be reduced if no more fuel is added to the fire... Remove the trigger if possible. ( The main trigger now lives somewhere else)
There is a lot of luck in it. Lucky to be born into good circumstances. Housing, disability, intellect, parents, grandparents, illness, disasters and life events.... All contribute. If you live in a noisy crowded busy environment it is a lot harder to self regulate and keep emotions within the ideal range..it is easier to be respectful if you are not about to meltdown or are not in meltdown and have now decoupled from the rational part of the brain.