I bolted the 2 sheets together before drilling the holes in an attempt to prevent any misalignment. Even so, it is tricky. Plans suggest using a drill press, but in reality humping 2 heavy sheets 4 x 9' around is really not possible. In the end I brought the drill press to the sheets, and then had to try and get it all level and wobble free etc. Infact unless you have a drill press with a deep throat, you cannot actually get near the second hole.
The white tubes are where the pivot pins will go in the final beam. Have to make sure the sides themselves are dead upright and parallel to each other first. otherwise you cannot get the tubes perpendicular and exactly level across the mold.as required. I struggled for several days before I was happy that all was OK.