In Odoo, the scheduling of resources for a project is based on the company that the project belongs to. To allow employees from both companies A and B to be scheduled on the same project, you can make the following changes in Odoo Studio:

1. Create a new company-independent project category: In the Project app, create a new project category that is not linked to any specific company. You can then assign this category to the joint project.

2. Remove the company constraint on the resource allocation: In the Project app, go to the "Resource Allocation" view and remove the company constraint on the resource allocation. This will allow employees from both companies A and B to be assigned to the joint project.

3. Add a new filter for the resource allocation : In the Project app, go to the "Resource Allocation" view and add a new filter based on the project category, so that only the joint project is displayed in the resource allocation view.

4. Modify the access rights: In the Users and Groups app, give employees from both companies A and B access to the new project category and the new resource allocation view.

5. Update the project form view: In the Project app, go to the form view of the project and add the new project category and the new resource allocation fields to the form view.

Keep in mind that this is just an example and it should be adapted to your specific use case.

It's also important to note that you'll need to test the changes before applying them in production as it may have side effects on other parts of the system.

Note: 

Editing resource_id domain on planning.slot.form through studio is not supported