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.
I actually have doubts that those two majors are "related". It sounds like a statement from someone who knows only a little about either. The thought patterns and insights are very different. It isn't a question about accumulating things, but about how the things fit together. 

Note that academia is about specialization - more and more the farther you go. But at the beginning it is natural (and good) to be undecided and hence flexible. 

You can do well (most places) with a double major but you have to work a lot (a lot) harder. Mathematics itself is a flexible position from which to go forward, as it is applied in many fields. The most flexible field (IMO) is actually philosophy, but that is a different discussion. 

I'll note that you don't have to decide early to accept one field and reject another (or others). If you develop a career you can branch out into other things that interest you. But you need to start from a solid base. 

But if you want to establish a solid base for doctoral study, a single subject with great grades would seem to be the best choice, by far. 

I don't know the rules in Germany for doing two MS degrees simultaneously. The thing I would suggest you be very careful about, however, is how much "double credit" you get for overlapping courses. If this is a common thing to do, then a given course may "count" in both degrees. Otherwise, you may actually be required to repeat things, or, more likely, to substitute elective courses for those you have taken in another degree. Just be sure of the rules and be wary about 25% "overlap".