If your are stapling you can make your scarf joint at the form and the staple will not only go through both layers of the scarf but also line up with the adjacent staple holes.

If you use scraps of wood for temporary backing boards to clamp against, wrap them first with clear packing tape for a glue release. Packing tape is also a good glue release on the edge of your station forms. Use the standard weight packing tape, not the heavy weight, because it also is a good stretch tape to assist clamping. 3M Scotch brand does not leave the PSA glue behind on epoxied surfaces.