There are already many buildings that segregate elevator systems into multiple stacked towers such as the Petronas Towers in Malaysia [1]. But in Petronas' case

There are already many buildings that segregate elevator systems into multiple stacked towers such as the Petronas Towers in Malaysia [1]. But in Petronas' case, it can become too complex that even the staff don't always get it right. [1] http://en.wikipedia.org/wiki/Petronas_Towers#Lift_system http://en.wikipedia.org/wiki/Petronas_Towers#Lift_system (note, lift means the same thing here)