Two distinct papers solving same problem to same conference, allow update post-acceptance?
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.