How do you write characters who speak over each other, or talk at the same time? Here are three ways:-
- indicate it in the scene description
- combine everything into one dialogue caption
- on Final Draft, there's a 'Dual Dialogue' option, which will give you this
All three are perfectly fine in my book because they don't confuse the flow of the script. For American scripts, where a Hollywood reader might unmercifully pounce on any formatting glitch, then it's probably best to stick to the dual dialogue option.