Advanced degrees in two subjects? Perhaps it is too early to ask this, but I will go ahead anyway, to satisfy my curiosity. I am currently studying Arts in a University, aiming to get a BA Honours in English and German. If I went ahead to get a Masters, would I have to choose one of them, or could I continue with both? And similarly for a Doctoral, if I get that far, does it have to be one subject at a time?
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.