When you use subcontracting feature in Odoo the standard route for the component it can be "Resupply Subcontractor on Order" what means you have the component in your stock and you will re-supply your subcontractor via delivery order (transfer). 

However, an alternative can be to generate a PO for the component in case you do not have enough stock via Drop Ship what means the supplier of the component will send the component directly to your subcontractor.

What is the current behavior that I observe?
When you have both routes activated in the product form only Drop Shipping is working what means that independently of the stock level, a DS will be created for that. You need to unselect "Dropship: supplier to subcontractor" to be able to resupply directly the subcontractor from your warehouse.

Expected behavior should be:

A- IF there is enough stock of the component to build that finished good, route "Resupply Subcontractor on Order" should take place--> a Delivery order should be generated to deliver from the company warehouse to the subcontracting location

B- IF there is NOT enough stock of the component to build the quantity of finished good for the SO, route "Dropship: supplier to subcontractor" should take place -> a new PO should be generated to buy the component to the vendor component and deliver directly to the subcontractor.

Unfortunately, both routes "Resupply subcontractor on order" and "Dropship vendor to subcontractor" cannot work together.

I would take into account how many times you will have to resupply your subcontractor or create a DS for a specific component, and based on that the workarounds are:

A- You just deactivate DS route for the component when you are going to resupply the subcontractor

B- You cancel the RFQ for your vendor component and create manually a transfer order to your subcontractor location