I wanted more consistent Z height so I made this to hold a small switch.
Nothing fancy but it works.
Groove on the back to key in to the vertical rod.
Two holes to fit some raw 1.75mm filament in to help hold the switch it in place. (not really necessary)
2 zip ties and a dab of hot glue. Glue just prevents it from sliding up the rod.
This was installed on a One-up but I don't see why it would not work on a Two-UP.

Switch used:
http://www.digikey.com/product-search/en?x=0&y=0&lang=en&site=us&KeyWords=sw503-nd
This is a common switch, used a lot in computer mice. Got a old mouse laying around? :)

G code to use only one endstop:
G28 Z ; Home Z only

See photo for wiring.
Ground pin is Z-stop Pin 3, the closer of the 3 pins to the USB port.
Signal Pin is Z-stop Pin 1, the farther one from the USB port.
Center Pin is not used.

Add "G28 Z ; Home Z only" to your start G code and you should be set.