There were in the past roles for a navigator and even a mechanic on board of airliners, but they were removed a long time ago. Now all the work is done by the 2 people in the cockpit and there is plenty for both in the take off, approach and landing phases.

I am not even sure where to put just another display in the cockpit that is already overcrowded. Or two displays, one for each station. Maybe it can be squeezed, but the efficiency may be zero or negative. Just imagine a plane is far more complicated than a car, having hundreds or instruments, gauges, buttons and lights. Even learning how to use all these takes tens of hours, adding more you can create an nightmare from a bad situation.

LE. There may be some solution, especially for modern digital cockpits, but there are hundreds of thousands of planes without a digital cockpit and with no way to upgrade them - they are not computers with standard interconnects like PCI, USB, COM and LPT ports, most smaller planes don't even have a standard cockpit layout and set of instruments. For example I piloted a few small planes of the same make and model, there were no two alike in the cockpit equipment.