A good full stack developer should 
    have double the salary of a frontend/backend 
    developer

1. While theoretically possible, it's uncommon (and given the rate of change and increasing complexity of both front end and back end stacks, unrealistic) for a "full stack developer" to have expert-level production knowledge in both domains.

2. Even if #1 was true for a given developer, they wouldn't have double the productivity (and/or billable hours per week) as somebody who was more of a front-end or back-end specialist.

It's similar to the reasons why, in the medial world, general practicioners (aka "family doctors") do not earn money that is the... sum of all other medical professions earnings. They are generalists . Their role is to solve many problems, but also to refer many problems to the appropriate specialist.

Even at the backend you have the application backend (c# or whatever) and then sql/rdbms. If you are that good perhaps you deserve thrice the pay by this logic ;).

Jokes aside, at the last good company I worked for where I knew other people's pay, I was compensated for the entirety of my skills compared to the good front end dev. We earned the same, and I was quite happy with that.

Jack of all trades...