Terms matter. When you say "parent case", it means any case that has a child case. Therefore, if I perform data propagation from a parent case (A) to a child case (B) that has another child case (C), then technically, B is also a parent case.
What you want to achieve is to propagate date between "top level" cases, not "parent" cases.
Data propagation happens only at the creation of the second case. So the way you configure it is the same. The difference is how you create the second case. If you want to create the second case as a "parent" case, use the "Create Cases(s)" step and select "Create a case" instead of "Create a child case".