As far as joining two sheets goes, I have a method that I really like.

Butt the two pieces together with the good side facing down, and sand the top of the seam. This is going to fill any spaces between to two with sawdust. Lightly blow off the sawdust on the surface (don't blow too hard, you want the dust in the seam to stay there). Now quickly run a bead of thin CA down the seam and immediately SAND IT AGAIN!

You will find that his method WILL ruin a sheet of sandpaper, but sandpaper is cheap, and it will give you a joint that is perfectly flat and will NEVER break.