This is actually not a new option and I didn't implement it. It has been around for quite a while but wasn't documented. I just thought it would be good to document it if it's present...
The description is mainly copied from the preexisting comment at the place it was used.

This option is actually only interesting for the y and y2 axis. It is not considered in the x-axis ticks computations.

Definition:
y=true, y2=false (default): y is the primary axis and the y2 ticks are aligned to the the ones of y. (only 1 grid)
y=false, y2=true: y2 is the primary axis and the y ticks are aligned to the the ones of y2. (only 1 grid)
y=true, y2=true: Both axis are independent and have their own ticks. (2 grids)
y=false, y2=false: Invalid configuration causes an error.