students - 2 majors with average grades or 1 major with excellent grades?

Is it incentivised to pursuit two (related) majors (eg. Math and Aero- Space Engineering) or a single major (eg. Math)?

The question is asked with the premise that if one chose a single major, one would have more time and thus achieve better grades (assume top 5 or 10 percentile), whereas with a double major, one would only be able to achieve slightly above average grades, but have a broader scope and be better prepared for industry (at least that was my line of thought).

I guess that the answer is very different for someone who wants to go into academia, as opposed to someone who goes into private industry, so I would appreciate a split answer regarding the two cases.

Thank you very much in advance.

Edit: With "double major" I mean two separate (masters) degrees. At least in Germany, that means much more work load: 1,5 times is an optimistic guess. This supports the premise that grades would suffer by doing a double major.