Both OSHA and NFPA 101-2009, “Life Safety Code,” require that at least two exit routes be available in a workplace to permit prompt evacuation (with the exception noted in 1910.36(b)(3)). Specifically, 1910.36(b) requires exit routes “be located as far away as practical from each other so that if one exit route is blocked by fire or smoke, employees can evacuate using the second exit route.”
Keep in mind that an exit route consists of three parts: Exit access, the exit, and the exit discharge.
If, for example, there is one aisle (the exit access) leading to the exit and discharge, would employees be able to access another exit and discharge if it were blocked by fire or smoke? If not, then an employer would have to provide a second exit access as a way for employees to exit the building as required above.