This describes a many to many relationship. While you could enter more than one ID in the same field if you separate the values with returns, this is not the best option to use in most situations.
Instead, use a third "join" table between the two:
Parent::__pkParentID = Join::_fkParentID
Child::__pkChildID = Join::_fkChildID
Your portal then becomes a portal to the Join table instead of Child, but you can include fields from Child in the portal row when you place the portal on the Parent layout. But you can also place a portal to the join table on the Child layout to list parents.
For an explanation of the notation that I am using, see the first post of: Common Forum Relationship and Field Notations Explained