Though this thread is a bit old I would add that it is possible to use dual port Ethernet with Rock
See this thread: ROCK Dual Ethernet - Primary Port Not Exposed [RESOLVED]
I’m currently using this config in my Mock using the internal mother board Ethernet and an additional usb Ethernet adapter. Easy setup and works great.
I see no reason it will not work also with the JCat cards.
As long as Rock/Mock sees the 2 Ethernet ports it can be configured.
It won’t be dhcp and the device connected to the second port won’t have internet access but as it will most likely be used for the Roon endpoint I would say in most cases you don’t care - all control can be through the Roon remote.