2 majors with average grades or 1 major with excellent grades?
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".