Thanks for tryout out this workflow!

I think we use workboards extensively, but this is the only company I've used Phab so I don't have a diverse sample set. We have workboards for subprojects, workboards for parent projects to view a team's list of new requests and high-level sprint plans (e.g. a backlog column, and a column for each sprint milestone). We click through on the milestone columns to view a sprint-specific workboard (typical Kanban columns).

I intend that it can not be in "Project X > Milestone 3" and "Project X > Project Y" simultaneously. Note that the parent projects are the same (both "X").

Regarding the expected behavior, do you mean it should not be possible for a task to be in both a milestone and subproject simultaneously?