Thibaut,
the driver, that you use in second step assignment, has to be dimensioned ALSO by dimensiones, that were used in first step assignment.
So in your case, AD Y should be dimensioned by CO1, CO2 and CO3.
How to expand AD Y into CO1: you need to write a IncludeCombinationParent(CO1,"all CO1 objects")
I hope it will help you,
Olga