Is it possible to make specific posts editable by all authors?

I have been using the RoleScoper plugin to allow certain people to edit a page or a post. You can also set it to let people (or roles) edit a page or post if it is under a certain category. It’s pretty good, and pretty easy to operate. You would just select the page/post/category and then select who has access to edit it (either my username or a whole role)

It is also free, if you want more features there is a paid version called “Press permit” although, RoleScoper has suited my needs well and the code is still kept up on.

I hope this helps