I'm planning to wire a few Arduinos together via RS485, and have one of them act as a hub that collects information from the others and presents it via http on an Ethernet connection.
Should be no problem.
The ETH shield uses SPI to communicate with a W5500 chip. That chip does most of the network processing for you.
The 485 shield just connects to the UART TX and RX pins. It's just serial communications with a couple of control pins from there on out.
Neither use any pins that the other one uses.