Routing problem with additional IP on different Interface with same gateway

[jonaskoenig]

Hey, maybe someone here has an idea.
I use opensene on a VM server as the router. The server is hosted at Hetzner, and I needed an extra IP to provide it to a host in the LAN/DMZ. So I ordered one. The problem then started as I learned that I could not route the IP to the same MAC address as the other. After this, I made another interface, second wan, with the mac address of the new IP and saw packages arriving at the interfaces. I should probably add that the new IP has the same gateway as the other IP, so I don't know how to configure the same gateway on two interfaces.
After trying out a lot of different rules and trying to capture the packages, I'm unable to get it to work. I do not know where the packages get lost. Maybe on the route back? Perhaps someone has experience with two separate interfaces using the same gateway and can help me.
Best regards,
Jonas