It's important to understand that you don't earn a salary because of your education. Education will help you get a job and it's that job that will give you a salary. Your education may qualify you for certain professions, such as an engineering degree being a component of the qualifications for employment as a professional engineer. But your post seems to imply that a degree in a certain field entitles you to a salary and that's a common fallacy.
It's also important to understand that you're not doing two degrees. I realize that some schools market this as such and may even hand out two separate diplomas. But unless you're spending eight years in school instead of four, what you're actually doing is double majoring. This will enable you to take the courses that will afford you all the privileges that come with a degree in each respective discipline - such as qualifying you for graduate school. But it comes at the cost of not exploring other subjects outside your field of study to the same extent. Electives in an array of other disciplines have been replaced by core courses in one or two other disciplines.
In terms of a career, you get paid for what you do. So, if you're double majoring in engineering and physics, you may have the opportunity to work as an engineer. In that case, you will earn the same as an engineer. Sometimes, you get a bonus for additional education if you have, say, a graduate degree. But no one is going to pay you more because you double-majored.
