Got a reply from Codesys support. This was a bug. It is fixed by upping Ethernet/IP from 4.3.0.0 to 4.4.0.0+. Now both can be added in a project.
Since migrating to Codesys V3.5.18 (64-bit) I am no longer able to add both an ethernet/IP adapter and an ethernet/IP scanner to a project. I receive three compile errors (attached). As soon as I remove either one of these the code compiles fine, meaning I can get an adapter to work or a scanner to work, but not together. The Codesys help shows both in a project, and it used to work on V3.5.16. Below is a link to the help page. I've tried adding another NIC and moving one to there. I got the same...