Using a long steel rule and a sharp hand held hobby knife has always worked quite well for me. Just like you I use masking tape to hold both joint edges tight together along the length of the joint, then turn the sheets over and open the joint into a V shape and run a bead of carpenter's glue along it and then close the joint and lay the sheets on a flat surface with adequate weights applied to keep them flat, wiping off all excess glue with a damp cloth.

Karol