multiple internal interfaces on same subnet

UTM 9.105-9 installed on a repurposed workstation-computer with 5 NICs
I started with 
1x external interface
1x internal interface
let's say I want to add another internal interface on the same /24 subnet,
but connected to a new switch , both interfaces would have to use the same dhcp server, same everything...

how can I do that?

Thanks

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