If you are interesting in dual-dialogue, here is a link to a FAQ about how to do that:
MULTIPLE SIMULTANEOUS DIALOGUE
Screenwriter can support more than two characters speaking simultaneously, but it cannot be done automatically.
Screenwriter has a feature that lets you turn off automatic formatting and use what we call "Normal Word Processing Mode" where you can write the format any way you want -- including multiple columns of dialogue and/or action. This works well for short sections of text but is not suitable for sections of text longer than a single page.
To do this in Screenwriter:
- Create a blank line of action
- Use the Format>Normal Word Processing Mode menu command to turn off script formatting mode and turn on the normal word processing mode
- Type the text exactly as you want it to appear (example below)
- Once you are done, use the Format>Normal Word Processing Mode menu command again to turn script formatting back on
Everything except the dialogue for Fred-Chris-Sam is normal script word processing. The three lines with the simultaneous dialogue is done using the "Normal Word Processing Mode" feature, which just turns off automatic script word processing.