Since firmware version 1.1.3 (07/2016) the firmware supports dual extruders with two actual hotends as well as only on nozzle and our y-piece.
Both options have their pro and cons. One nozzle with the y piece is much simpler to install and operate.
In any case you need two extruders, one of them should be mirrored.
one nozzle, two extruders
Easier for beginners. Read more: Dual Extruder single nozzle
two nozzles, two extruders
For more advanced Users. Read more: Dual Extruder dual nozzles