technically there is something very similar in operation now..think about the Tower Of Terror  (opened in 2004 DCA and 1994 in WDW)  ride at Disney California Adventure...it has 2 "elevators" that can operate inside one elevator shaft..at the end of the ride it takes one elevator container out of the shaft (moving sideways)  for loading and unloading ..while that is happening another elevator container is put into the shaft and moving up and down in the shaft..