You can add multiple connection inputs to a form so that changing the record of the parent will update the records available in the child.
You want to assign a Project record to a Company and an Employee. As you select a Company, the available Employee records should change by only showing Employees connected to the selected Company.
Note: The Show filter only works with "one-to-many" / "one-to-one" connections. "Many-to" connections will not be a filter choice.
This requires 3 objects:
Many Employees connect to one Company. Projects have two connections: one Project connects to both one Company and many Employees
For a form adding a new Project, add inputs for both the connections:
You can then edit the child connection (Employees):
And you'll see an option to only show the Employee records connected to the form's parent:
When the user selects a different company the employee records will automatically update to only show the employees connected to the selected company.