Hello,
with Powerlink it will always be a asymetrical set up, because it is a Master-Slave Fieldbus.
One PLC must be the Master, and control the Pumps. While the two PLCs communicate to each other and transfer the Request to the Master PLC.
If you want to generate a symetrical setup where you can use a Machine-Program on both PLCs you will need to use a different communication, where no fixed Master-Slave connenction is given.
Instead of a BusCoppler i could imagine that the Diaphragm Pumps are connected to a small PLC X20CP0410.
This would allow Protocols like OPCua C/S or coustom UDP Communication.
I think the main PLCs are allready connecten via TCP to allow a online connection, so you would only have to add the third PLC to the Ethernet.
Just to have a look out of the Box.
OPCua C/S and UDP are not real time, so it would only work it there is some feedback channel and the other machines can wait for the signal.
Greetings
Michael