Thanks a lot dear gentlemen for your quick replies. My project is still in construction stage. I've consulted an elevator supplying co. and according to their engineer, it is not possible to operate one sided at a time. He is telling that when the elevator will arrive on a call from any side, both doors will get open if both doors are at the same level. If there's a difference in level say 1 to 2 feet then only it is possible to open the door of the caller side only. Other door will remain closed otherwise not.