Already responded to your other comment first. But no that wouldn’t be difficult. Think about the screen.

If you long touch the top of the screen, there’s no Taptic Engine there. But you still feel it. The entire body of the iPhone conducts the vibration no matter where you actually touch.

The button itself needs to be capacitive to interact with the capacitive buttons. It could also just be straight up metallic with some sort of non-scratch bottom or just a thin enough plastic coating so as not to insulate the button. But the button would happily conduct the vibration of a second Taptic Engine is used as long as it’s in contact during the feedback tap. But I’m not convinced that’s actually necessary as one engine covers the entire phone for screen presses anywhere as-is.

They could certainly fit another engine in if they want to increase the strength of the haptic feedback or add more complexity, but it wouldn’t be strictly necessary. I’m more of the opinion that every cubic millimeter possible should be devoted to battery gains.