Put the two edges side-by-side to be sure they mate well. If they don't you can sand them, or slightly overlap them and (using a straight edge) make a single cut with a razor blade through BOTH sheets so the edges will mate well.

Now lay them side-by-side and tape them together.

Flip it over, bend them at the tape joint and apply glue to both edges. Wipe off any excess.

Lat the sheets tape side down on a flat surface, again wipe off any excess glue, lay a sheet of waxed paper over the seam and then lay something flat over the seam (Like another piece of sheeting, or a long straight edge). Finally, add weights to hold it flat until the glue dries.

If you want, you can even leave the tape for a little added strength.