When each server has a different function and role, then you need to link them together for communication with each other. Using P2P Group can help you to create the link easily.

Appoint one Farm Server, which all other servers can connect to. LAN group with server computer is called Server Farm, and the term Farm Server came from this.
All other servers connected to Farm Server becomes Farm Client.
Each Farm Client communicates with each other by P2P.

At this circumstance, each Farm Client, meaning each server process can directly communicate with each other no matter it is client or server. Thus it enables to communicate in net without a certain form and is independent from each other.

Server-to-Server communication topology

We recommend you to use LAN communication between servers for server communication.

In server communication, Limiting the size of communication message has to be considered.

The sample for creating server communication can be found at Sample code for Distributed Lobby System.