If the two controllers are mounted very close to one another (within 6 meters), and use the same power source, then you don't really need isolation between them, so you could just use a cable (like the 1747-CP13, I think) between the two DH-485 ports instead of the 1747-AIC isolators.
Elsewhere on this forum you'll find a rant from me against doing that very thing, but it's not a big deal if you don't have a difference in ground potential between controllers.
Maybe there's something I've missed about what's occupying the DH485 ports on these controllers.
You have to configure DF1/IP address equivalents in a routing table inside both 1761-NET-ENI's to message between controllers using those devices. Messaging on DH-485 just needs local address numbers.