There is no way with a serial bus that you can talk to two devices at the same instant. The device sending a message streams a sequence of bits along the wire, it cannot send a stream of bits to one device and a stream to another at the same time. It is, of course, possible to send a message to one modbus device and get a reply, and then send a message to another modbus device, and then one to a third device, and so on.
Of course it is possible to communicate with more than one, but each transaction must be complete before the next is started.
Please describe exactly how you are trying to communicate with two devices. Select the nodes that do the communication (only a few nodes, we do not need the whole flow), use the menu Export facility to export to the clipboard, then paste the export here. In order to make code readable and usable it is necessary to surround your code with three backticks (also known as a left quote or backquote ```)
```
code goes here
```
See this post for more details - How to share code or flow json
