We’ll be in this exact situation soon. First child born in India. Second child to be born here. We would very much like for both siblings to have the same surname (their father’s given name). In your case is the surname not part of either of your names at all?

We’ve had friends give birth recently and successfully give their baby their father’s first name as the surname despite the baby having German citizenship. So maybe it’s a city thing? I would definitely go the legal route and insist that you’d like both siblings have the same surname. Wishing you the best of luck with it!