To create simultaneous, or "dual" dialogue, add a carat
^ after the second speaker's Character element, like this:
SPEAKER 1 I'm talking! SPEAKER 2 ^ Me too! At the same time!
While editing, the Dual Dialogue pair will appear indented correctly, but won't be vertically sandwiched together. To see Dual Dialogue as it will appear in print, Preview your screenplay.
To revert to normal Dialogue, delete the carat.