From personal experience, I can tell you that when you glue two pieces of leather together, the absolute best thing to use is Contact Cement. Coat both pieces, let it dry til tacky, and put them together and use a roller to apply pressure to get them to fuse together tight. Aerosol glues, while convenient, do not hold two pieces of leather together nearly as well. The best of the aerosol glues is 3M high strength Super 90, followed by 3m Super 77. After glueing, you still need to stitch along the edges. I use the spray glues when I glue cloth to leather, and iron it afterwards to really set the glue. Works very well. On my projects where I have spray glued leather to leather, it simply doesn't hold together as well as contact cement. Yes, contact cement application is a pain in the ass, especially a large area.