Today i figured out, that there is already a possibility to add a second DHCP Relay. Just create a "availability group" in "Definitions" and add two DHCP Servers like Windows DHCP servers. Thats all.
Availability groups are rather new, so I think that's why no one offered that as a solution. Well, that, and I think we all assumed you wanted different relays for different interfaces - shame on me for not asking!
Thanks for sharing your solution with the rest of us.