Advanced degrees in two subjects?
If you do research based Masters and/or PhD, you could probably find a specific research that combines elements of both - quite often (not always), these types of projects turn out to be the better ones, as they force the researcher to establish, rationalise and justify the links between them.

Even though my research has not been in linguistics, rather in the applied physical sciences field, my research combined a few subjects together - in my case: atmospheric physics, programming and signal processing. 

So, it is very possible to do, in your case, relevant elements of both English and German in a single research postgraduate degree.