Help!

Is there any reason that I could not control 2 machines from the same set of drivers by switching the output from one to the other through a serial data switch?
The switch has 9 pin sockets and I'm connecting two steppers, therefore 8 wires through each input. I am aware through previous bad practice that I need to power down the drivers before switching.
Is there also any reason that I need to have connections made to all three outputs whilst operating the system. ( my lathe has 2 axes, my mill has 3)

Ian Hart