I have a zero-lotline situation that requires some creative thinking. I need to achieve a 2-hour fire rating for a framed 2 story building's east and west walls which have neighboring structures on their respective lots. Architect drew the walls as double thick 2x6 with 4 layers of 5/8 firecode gypboard. I am trying to get same with one 2x6 wall. Is cement board fire-rated for use as a sheathing? Any other ideas (masonary is being considered).