layer or coating

Hi everyone,

I'm new to Illustrator and using it for scientific visualization.

I need to draw a long blue waveguide (a rectangle). At one spot, I want two coatings on top of the waveguide (gold + another material). No layer under the blue bar. I attached a reference image done by AI.

What’s the best way to stack these layers with different thicknesses so they sit smoothely on top of the eachother? Any simple steps or tool suggestions are appreciated.

Thank you.