one is willing to pay for a good is equal to the utility provided by that good [...] then I value the coffee twice as much as the other person > And because this assumption is so fundamental to economics,

That's just plain incorrect, it is not. It might be one of the thoughts that has been used by some economists some time. It is far from any bedrock economic theory.

Econ 101 is the most I've ever studied the subject.

Well there you go, that is the reason you would think that.