When I built the sub floor for the bathroom, I forgot to compensate the thickness of the hardie backer that I will be install on the hall way floor . Once I install the hardie backer on the hall way, then it will be higher than the bathroom floor. So can I add another hardie backer on top of the hardie backer in the bathroom floor to make it level with the hall way floor ? Thanks

Yes. Lay it the opposite direction from the first layer so seams don't overlap.

OK Thanks a lot your help