Sometimes you write dialogue so good, that two characters need to say it at the same time. In your script, this is accomplished using dual dialogue. Here's how to format your double-speak in Scrivener, Final Draft, and other screenwriting software.
Hello, Character. It's me, your writer. First, let me say that your dialogue sounds snappy and authentic. Awesome. But, there is some stage business I need you to do, and it needs to happen right here. Yes, it's important. You can keep talking as soon as you're done. Thanks, I appreciate it. Okay, that's all I needed. Carry on.