Wireshark capture between two Modbus Serial devices

Out of the box it can't. The usual way to monitor such traffic is to convert the serial traffic to Ethernet using a device server such as these Lantronix devices (there are other similar, cheaper, devices available). You can then use one device at each end and use a tap or switch with port mirroring (or even a hub) in between each end and then capture the traffic.

If the Modbus master is on a PC, the application can often be configured to use TCP/IP instead of serial, in this case you only need one device server at the Modbus slave end.