Well, it probably depends from the PLC's involved, but also an old hex or even a hex lite or hap (these can be found used and even if new the hex lite costs 40 bucks and the hap 45) would most probably do.
Since it probably is "internal" only even running RouterOS 6 would do.
Just in case:
HEX Lite for routing between subnets
Port based Routing to 2 identical IP