The Magus

Hello all, I am a M.A. student in musicology. I will apply to Individual PhD's in Europe (particularly Germany) for Fall semester 2018, but I couldn't find an answer on the internet for one particular question. My PhD project focuses on computer music and posthumanism. So, I am looking for a supervisor in both musicology and philosophy fields. I wonder if I find a supervisor in the philosophy department, would I face an obstacle when I am registering to the university. It would be a nonconsecutive PhD and I thought maybe they'll want a diploma in the same field. I want to know what is the standart expectation of the universities and learn if this change in the field is possible. Thank you very much.