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:
DANIEL
I don’t care if she did say that.
DONALD
(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.