Joining the Face Board
  About joining the face board (plywood, OSB, etc.) Over the years, I have tried joining the cornice faceboards in various ways. I used to use mending plates as Agnes said. Since then, with time and experimenting, I have settled on just butting the pieces together and  running a row of staples up and down both sides. I staple it so that one leg of the staple goes into each piece being joined, like this

I put the staples close together, as shown up and down both the front and the back. I put more staples near the bottom than at the top (because the headrail holds the top in place. At bay joints, at the corners, I have used webbing, which leaves a flexible joint (which is helpful for the installer if he needs to adjust it a little.)  The thing to keep in mind is that the headrail holds the facing in place, and no is walking and jumping on these cornices. (at least I hope not! )

Try it!
Now, before you scoff at this, actually test it out. Fasten two faceboards to a headrail and then join them as I had described. Then test the joint. If you did a good job of stapling, it should be solid.  I have joined many like this and never had a problem. I've never use the pressed paper, so don't know how it would work with that, but I would certainly test it out before deciding whether or not to use this method.