In the real world, people do not usually wait for others to finish talking before they speak. People talk over each other, over-compensate with their voices and even talk about completely unrelated subjects while the other person is still mid-sentence. This is fine to add to your screenplay as well, especially if a character just begins before the other one is quite finished.
There is a simple standard that is used for “overlapping” dialogue, where one person’s lines and another person’s at one point are occurring simultaneously.
When you are planning to have two characters overlap their dialogue, you begin by writing the first character’s lines out completely untouched. The first character does not need any specific formatting as the second character is going to take the brunt of this. When you write out the second characters lines you are going to indicate this overlap in the parenthetical instructions you give to the actor.
This would be things like the movements that they do while they say the dialogue or a fashion in which they speak. Here you would instead just put “overlapping dialogue,” or a comparable statement. Example:
I don’t care if she did say that.
(Overlapping) If you don’t care then neither do I.
This will make the last few words of DANIEL’s line will happen at the same time as the first couple words of DONALD’s line. The level of overlapping will be up to the director, but you do not want too much otherwise the lines will get lost completely on first viewing.
There is another way of doing this using subtitles, where the subtitle format is used to indicate overlapping dialogue. When you are writing subtitles in a script you write parallel columns where you have the original line, and then the language translation is on the right-hand column.
You can do this to indicate simultaneous dialogue as well. Yet, this can be confusing to most readers and is not advised. There are a number of alternatives that are conventionally used in live theater, but none of them are used common in screenplay writing.