Why not use a USB3 Ethernetadaptor and you will have your router
Dilebus suggestion is great.
Ubbe, I heard Ethernet on USB consumes a lot of CPU.
What about installing a NIC on a PCI Express* slot ?
Would also be interested in dual ethernet NUC!
But in the meantime, most beautifull solution is vlan tagging (802.1q) who seems to do the job (you'll need a correct switch to do that).