How do you add two utilities together?

They are numbers. Add them.

So are the atmospheric pressure in my room and the price of silver. But you cannot add them together (unless you have a conversion factor from millibars to dollars per ounce).

Your analogy is invalid, and in general analogy is a poor substitute for a rational argument. In the thread you're replying to, I proposed a scheme for getting Alice's utility to be commensurate with Bob's so they can be added. It makes sense to argue that the scheme doesn't work, but it doesn't make sense to pretend it does not exist.