To use Dual Dialogue in Final Draft Writer to show two characters speaking at once, you first need to have a Character Element followed by a Dialogue Element immediately followed by another Character element and Dialogue element:
Once your elements are correctly positioned, click your cursor next to the first Character element. The two dialogue bubbles in the floating toolbar will color in:
You can then click the dual bubbles or press CMD+D (Mac) or CTRL+D (Windows) to dual the dialogue.
You can edit your dialogue while it's still dualed by clicking into it and typing.
To undual the dialogue, click your cursor into the dualed dialogue and click the dialogue bubbles or press CMD+D (Mac) or CTRL+D (Windows).