When in an emergency, one would try to call attention to his/her vessel,
so
you need to do something that does not make sense or seem ordinary.
Flying a "Yes" and "No" flag at the same time is certainly not ordinary.

That makes much sense.
I have asked the DSV (German Sailor Organization) now too. Probably they can
approve this. I will post their answer here.

Thanks so far for all hints.

--
kind regards,

Jonas Stein