I would suggest to check this configuration example.
The idea here is that one of the RUT240 acts as a main device with DHCP server, switching capabilities and, preferably, provides internet access, while the other is a client to the first RUT's access point and rather provides access to the wireless medium to the devices connected to it, also connecting those devices into a single LAN network.
In your case, you will probably need to install Relay configuration package from Services -> Package manager in one of your RUTs, which will act as a wireless client. Next, configure second RUT as a wireless client to the first. Then in the same device navigate to Network -> Interfaces -> Relayd (firmware versions 7.3 and above), Add new instance by selecting lan interface, then assign wireless client interface.
Other than that, if your PLCs have static IPs, they will both have to point to the same RUT IP with DHCP server enabled as their gateway. This should be enough for the PLCs to communicate.