Different IPs are already set through drivers that you add to the form.
If you are using the same driver to communicate to 2 or more different PLCs then just add more instances of that same driver to the form and set different IP addresses.
To have a single button click event communicate to multiple PLCs, you could set communication to one PLC through PLCAddressClick in the properties window of the button (using current CommComponent assigned to the button) and communication to the rest of PLCs through the button's Click event which you access by double-clicking the button in DesignMode.
Then just add code, similar to what Archie already explained in another post:
EthernetIPforCLXCom2.Write("MyStringTag","ABCDEF")
EthernetIPforCLXCom3.Write("MyStringTag","ABCDEF")