I finally have solved my problem after more research!
Main idea: Event Hit - velocity before hit - World Creation - Unreal Engine Forums
(Answer from Doug Wilson)
I have to “Add Component: Collision”(in Viewport of my Blueprint) that is slightly bigger then my mesh collision. In my case a Box Collision does fine. I called it “OverlapableComponent”(see EventGraph). After that i added a new Object Channel “VehicleOverlap”(Edit → Project Settings).
Now i set the Object Type of my newly added Collision Box to “VehicleOverlap”. In my case the Box should only overlap with other “VehicleOverlap”-Objects. If you dont want to accomplish this, you might not need a new ObjectChannel.
And that’s how i solved my problem in Blueprint (You can ignore the Print String, just needed it for testing):
Now two or more cars can hit one car at the same time and the HitVehicleVehicle function is only called once each time a crash happens. You might not need the BeginOverlap Event. I needed it because i had to get the velocity of my car before the OnHit Event happens. EndOverlap Event resets the possibilty to crash a car again. If someone needs help, i am always open for questions.