MBot Grid II Dual - nozzles offset and Slic3r

I would like to try to generate G-Code with Slic3r and I need to know the exact offset of the second nozzle. What is the 0 reference in dual printer? Right nozzle, left nozzle?
I’ve managed to correctly print with one head, by using start / end G-Code as suggested in one other topic. What should I use to use both heads?