General
I have twice faced the same problem. My solution was to complete the claims for all the possible insurers (in one case there were three: a flight specific insurer, a credit card insurer and a general travel insurer). On each claim form I gave clear information about the other two insurers, including their contact details ... and I was fully refunded for the value that I had lost.
I figured that the insurers themselves were more likely to be able to work out an appropriate apportionment of their respective liabilities than I would be. After all, they deal with insurance every day.
Country specific
I do not know the law in Germany but in Australia, insurance is dealt with under Commonwealth ("Federal") law. The Insurance Contracts Act states the explicit assumption that all parties involved in an insurance contract will deal with one another with the utmost good faith. I do know that such provisions are not uncommon in the law of other countries. The assumption (enforceable at law) is that, given your own full disclosure, the other parties will deal with you fairly.
Insurance generally
Again, under Australian law but also a feature of other countries, is that insurance by its very nature, relates to a risk. One cannot insure for more than the risk; hence one cannot, or rather one ought not be, refunded more than the value lost. Insurance is not a profit making exercise for the insured person. But there is nothing in that presumption, nor in the article of utmost good faith, that would prevent you, like me, from lodging claims with all parties, making full disclosure about the state of affairs, and letting it play out.