hm. let's say both company share one location, and therefore one warehouse? Would it be a way to create two ex-factory warehouses (where in the logic of odoo the manufactured goods will be put after manufacturing is finished) and use one of those to assign a Sales order as desired?

How do these two separate companies share the space? The two locations could be 'Warehouse - West' and 'Warehouse - East'. Two separate companies usually don't share a space without an agreed delineation or boundary.

in my case these 2 companies are very close and actually share one physical location. Setting up these 2 separate ex-factory warehouses would mainly be a virtual thing in order to achieve my goal to assign a SO to one of the 2 companies

still struggling because the I can't freely choose a company when creating a new warehouse. the field "company" is fixed to one of them (the company that was set up first)

From the user preferences dialog (accessed from the dropdown arrow next to your username in the top right corner) change your company to the one you want to create a warehouse for.

played around with this a little now and it looks to me as the sytem does not care which company owns the warehouse related to an SO.Furthermore the system seems not to care which company the customer is related to. Looks like the user that is doing the SO has to be user of the company you want to do the SO with (in the moment you create the SO). And you can switch that from the user's Preferences. Not sure whether I think that is any convenient or usefull method. Nevertheless it seems to work, so I guess you should just go with that. I would find it just much more logical whether you could simply choose the company when you create an SO.

The systems supports a hierarchical (if needed) multi-company configuration that allows users with the correct permissions to create and modify documents from other companies. I suggest you post another question with what is missing in your understanding of how to setup multi-company for your scenario. The company property of Sales Order is separate from the Warehouse/Shop and means only that the order was created by (and will be managed by) a specific company.