I highly recommend to go through the NOW Learning modules for Order Management which covers this topic.
The staggered approach is only applicable if there is a decomposition involved and this will be subject to your Catalog Definition and Requirements.
Assuming, there is decomposition involved you will need to define a char on say the parent CFSS.
On the Spec Relationship between the CFSS and RFSS, you can define a decomposition rule such that the decomposition happens only if a value is set for the char.
The value for this char would need to be set via the subflow by updating the Order Char record for the domain order for the parent CFSS. This step can be done after the External API call in your example.
Once the char is set, the decomposition condition will be satisfied to create the domain order for the RFSS, invoking the associated orchestration flow.
Note: Defining a Char on the CFSS (or rather any Spec) just to control dependencies is not the best way of modelling as the Catalog Definition and hence the Inventory records get polluted with unnecessary data that is not relevant beyond it's temporary purpose. As such, consider the approach with overall implementation in mind.
