Hi there! DanMe.... Hello also, to all the Ladies and Gentlemen, who offer so many wonderful insights on this forum.
In regards to how you would format dual dialogue,take a peek at the Page2Stage website. Hey! I'm not spruiking the product. Okay, maybe just a little.
Page2Stage is free,and contains many great functions,including the dual dialogue functionality you desire.
The downside is, they no longer have any "Tech support" available.
If your in a hurry,I'll give a brief explanation.
The characters speak in left and right downward columns , opposite each other.
Your action(descriptive passages) would simply be placed above or below the dialogue(s), as per standard formatting.
I have never heard of dual action before,though I feel you meant,formatting dual dialogue with Action(descriptive)passages.
I have only seen dual dialogue, from time to time.
It is used in plays,though in regards to screenplays, as Nikee Goddess stated, the usage of dual dialogue would not be common place.
Though, I am sure there are exceptions,where using dual dialogue, would be acceptable.
To PLOT DEVICE.
I have a trial download of Final Draft.
I tried out what you suggested from the Final Draft 7 help file,regarding dual dialogue.
I thought this may solve DanMe's issue,though I ended up with a dialogue box stating that "I was placing the curser in the Character Element,and that this was incorrect".
Oh well! it is a trial download afterall.
Good luck.
Daniel.