How-to dual extruder

[marko001]

Hi all,
sorry if this isn't the right area to post, in case please admin move it.
I would like to buy a second extruder for my printer.
I exactly need to know what I have to buy (and to modify to firmware) to have it working.
I suppose I need:
MK8 Dual extruder holder
New MK8 extruder

Anything else?

Just for info I have GT2560+4A4988 Control Board and PSU 110v-240v Dc12v/0-15A

Thanks,
Marco