I'm starting to see a pattern here - I think,
If it fair to say <mọi | cả hai><subject>đều<action> is always the correct sentence structure?
yes, it is a correct structure in Vietnamese
If only this happened in Romeo and Juliet...
You should have a lesson/tip explaining how using cả/đểu together is the best grammar construction.
Why is "deu" needed here when the idea of "both" families is already indicated in cả hai?