This seems to be a pretty standard way of doing formal debates: http://richarddawkins.net/forum/viewforum.php?f=23
That would be quite easy to do. Just get two people who either disagree on a subject or wish to take a different stance on a subject and then let them post alternately.
It would be moderated (i.e. posts would be checked for possible inflammatory remarks, issues of ambiguity etc.) before the posting goes live.
Obviously exchanges like these would be slow, but (hopefully) of high quality.
But there are other ways we could do things as stated above.