Agreed with what everyone said above about perfectly flat mating surfaces.
I like to get mine perfectly flat, then sand them lightly in a cross hatch pattern with 320. I glue them with Devcon '2 Ton' epoxy. I've got laminated calls over 20 years old that have never delaminated.
One final detail I've found is that certain woods laminate well, and certain woods will have tearout at the lamination joint. You have to experiment to find the best lamination woods. Two of my favorite are Bois d'arc and Walnut.... Hmmm Maybe I need to do a new lenghthwise laminated call in that combo....