Maybe too late, since you bought the other stone, but I use firebrick splits for pizza all the time, and have used them once so far for bread as well. It worked out great! There is a pic in here:
New Braunfels Bandera, 22.5" WSM, Weber Ranch Kettle, 5 or 6 other Weber Kettles, Party King tailgate grill, and a modded "pizza grill" that is falling apart.
Self-proclaimed President of the Canton, OH chapter, Weber Rescue Society.