How can two people write a screenplay together?

Is there a specific method for two people writing a screenplay? Me and my closest internet friend were planning on writing one together, but due to the inherent difficulties in two people writing one script, he's 2/3 of the way through and I've barely done anything. I'm trying to imagine how Matt Stone, Trey Parker, and Bobby Lopez all wrote The Book of Mormon together, but I can't picture it.

How does this work?

The key here is the same for all collaborative projects- communication. Set aside a specific time every week or day or whatever fits into your schedules and use that. Set up a Google Doc so that you can watch changes get made in real time, then text or use AIM or Skype in the meantime so you can discuss.

Whatever you two are doing now obviously isn't working, since one is 2/3 done and the other hasn't started. Communicate and make sure both of you are clear on what each of you want for this script, and it should fall into place.

There really is no "one" method for collab projects. How you and you friend write your screenplay together really depends, more or less, on how you collaborate to do stuff in general. Do you each take a portion of the work, so you end up with a situation where you both physically write portions of the script? Do you each do your own thing first, then take parts of each? (Two separate first drafts, with the best parts taken from it?) Is one person the "ideas" person while another the "worker" who fleshes out the small details? Crystal's right, in that communication is important. You need to talk about your ideas with each other. But otherwise, it's really just a matter of finding a way of putting that screenplay down on paper that plays to both your skills.

As an example, I've got a co-writer on my Wordkeepers project, and the we we do it is, we pre-plan the story ahead of time in big brainstorming sessions. We also come up with things we want to address in future sessions or in later books. Basically, we just sort of spew out ideas. Then, because I'm the more technically advanced writer of the two of us, I actually write the scenes and flesh them out with appropriate details. Then we read my drafts and discuss and critique them. That's our method, if it helps, but your method should be what works best for you.