Two distinct papers solving same problem to same conference, allow update post-acceptance? Authors A and B both submitted different full papers solving the same Computer Science problem X to a prestigious conference and were accepted. They each have clearly distinct approaches, studies, and novel contributions. The conference camera-ready deadline has passed, but the conference has not happened yet.

Is there any known precedent to deal (or not deal with) situations like this? Should special provisions be provided to allow the authors to update related works to reference each other?
Sounds like a pretty straight-forward case of simultaneous invention.  

There's nothing wrong with these papers being published without reference to one another, just as though they had been submitted to different venues.  At the same time, if the conference wants to arrange for the authors to add citations to one another (as would likely happen if they were to be published in the same issue of an journal), then that would be a neat and elegant refinement that would probably benefit both sets of authors.  Moreover, the associated society publisher (e.g., IEEE, ACM), is likely to be able to handle arrangements without much trouble as long as they aren't too far into production of the proceedings.  Don't do it if only one set of authors can update, however: an asymmetric arrangement would inappropriately benefit one side over the other.

Thus, I would recommend making the extra effort if it's within reasonable bounds, but not going to heroic lengths if it will badly complicate things.