Configure ramps+marlin for dual Z driver stepper and dual Y driver stepper

Hi I build a large printer, X:500mm,Y:600, Z:800 with two NEMA 24 motors in Z axis
and two NEMA 23 in Y axis.

I want to add a extra driver for the second motor in Y axis.
NO want series or parallel conection.

The extra driver will be over a
KEYES A4988 Stepper Motor Driver Shield

I read this article to add a third extruder on the AUX-2 RAMPS ports
[reprap.org]

So i want to redirect Y axis to AUX-2 ports.

Which are the changes that must to do in marlin firmware?
There is another better alternative?
Greetings

Edited 1 time(s). Last edit at 06/05/2017 02:17PM by marceloarguello700.