How often do mids graduate with a double major or a major and minor?
It happens but not often. Mids are already taking 16-20 hrs/semester just to complete one major. It's nearly impossible to add a second without having validated a LOT of courses.
In order to do these do the 2 majors or major and minor need to be similar?
Depends. I believe they offer language minors now. If you can validate 2 yrs of a language, you could probably use your humanities distribution requirements to earn a minor (or get very close) in a language, even as an engineering major.
Other than language, they would have to be similar. You might be able to do two humanities majors because they often have similar core requirements and you don't have the labs, which eat up your hours. I think it would be very hard to do a major and minor in the science/math/engineering disciplines. Not saying it can't be done -- just hard.
How many courses would you have to validate plebe year to do this?
Depends on what subjects you want to major/minor in and what courses you validate.
Have you ever heard of someone getting a double major and minor?
My brother did it but he was at a civilian college (majors= economics and math, minor = comp. sci.)and I don't know if that will work.
Your brother did not have to take all of the other courses you need at USNA that do not count toward any major. These include seamanship, leadership, Law for Junior Officer, etc. Also, he likely did not have the general core requiements that may not count toward YOUR major (i.e., if you are a Poly Sci major at USNA, you still take calc, chem, engineering, physics, etc.). While most civilian students have SOME distribution requirements outside of their major, not nearly the number you have at USNA. For example, I was a Poly Sci major, but over half of my credit hours were not even in the humanities (let alone PS).
If I got my wish I would want to do General Engineering with a Chinese Minor (which according to the website shoulb be coming soon,) which I know are kind of different.
As noted, this MIGHT be possible.
BTW, I believe that most students who validate a number of courses elect to pursue a master's degree while at USNA rather than add to their majors/minors list. This program allows you to take courses at a local college your 1/C year and, I believe that in most cases, you can stay at USNA for up to a semester to finish your degree.