Integrating code for embedded systems
How do 5 construction workers go about building a house?  Do they simply build pieces independently and hope that the general contractor can fit the pieces together?  Of course not.  What really happens is an architect makes a blue print and the workers build to the specifications in the blue print to ensure the pieces fit together.  For software, the system architect makes the system design and the developers build their pieces to fit the specifications of the design.  The system design should include at least an overview of how the pieces will integrate, such as APIs.

Choosing whether to use a multitasking RTOS is like the construction workers deciding what building materials to use.  It doesn't really matter how many workers there are.  It depends on what the architect (system designer) specifies.  And the architect makes that design decision based upon the product requirements and constraints.