A double decker bus has a very similar setup to a normal bus. The main difficulty in the double decker setup is the player and passenger navigation trough the bus.
Try to approximate the stairs to the upper floor with Move Nodes. Passengers follow the lines drawn by the debug visualization if there are not enough nodes to approximate their path they will clip through geometry.
Make sure to set the Node Type of the stair Move Nodes to “Walking“, otherwise some passengers might stop at the stairs, blocking the path for other passengers.
Sample Move Node setup on stairs
The player navigation in the bus is defined by the physics asset of the bus. The collision setup can normally be fairly accurate to the mesh, but as the stairs in double decker buses can be fairly cramped you might need to give the driver character more space than normal to be able scale the stairs. Keep the paths a bit wider and be careful with the ceiling colliders.