I agree, and I had tried it, but didn't work. Specifically, I tried (among other combinations) for mac inputs, these.
Gateway(=router on mac) 192.168.100.10
for the 2nd SM ethernet input connection to mac, called 'LAN 2' by the computer.
And in Ekos profile used 192.168.100.20 (and also tried 192.168.100.10)
and it didn't work.