Same person as supervisor/co-promotor and professor/promotor
Considering German institution/university where I am doing my Ph.D. now, you are the person who is responsible for finding a second supervisor to your thesis. 


Regulations for Doctoral Degree Studies in Germany says (computer science field):


§ 3 and § 4 § 3 II (Primary Supervisors):
“Professors and lecturers who have completed their post-doctoral qualifications, are members of the faculty, or were included in the acceptance of the dissertation, can be primary supervisors.”

§ 4 II (Secondary Supervisors):
“Secondary supervisors assume a supervisory and consultative function. Professors and university lecturers with post-doctoral qualifications can serve as secondary supervisors. The Doctoral Committee can authorize additional scholars and scientists to serve as secondary dissertation supervisors upon case-by-case assessment and the issuance of a resolution to that effect. They need not belong to the faculty.”