It is not possible to restrict the list of values that appear for the same eunmval field in object Y. You could use a calculation to SET the same values on Y that are set in X, but then the field is not editable on Y. I assume you want the user to be able to pick all or a subset of the values selected on X?
Another option would be to use a CALCULATION to set the values on the same ENUMVAL on Y, but add a view rule to not display this field on Y. Add a second separate ENUMVAL field for object Y which has the same values as the other ENUMVAL, and setup a dependent picklist to only show the same values that are selected on calculated field. This way, you only see the subset from X to choose from.