How do you format two characters talking at once?

When two characters are talking at the same time, it is referred to as “dual dialogue,” and the two speakers’ text blocks go side-by-side.

Most screenwriting programs have an option for this. In Final Draft 8, for instance, if you type the dialogues normally with one below the other, highlight both, and select Format –> Dual Dialogue, it will put the blocks side-by-side.