The two sites would elect to use one of the PCs as the “server” and the other a client to it. You could consider making both servers with one as a backup using conventional networking to back-up each other’s files. It comes down to how the IT set-up is done.
For operating two ECS clients and on-line there are two scenarios:
1. One ECS PC has the configs opened and ECS Online is on with system 1, then the second ECS PC can only open a read only copy of the configs from system1. Then with either user selecting system 1 with ECS online mode on, whatever changes are made from either PC is reflected on the other PC.
2. One ECS PC has System 1 config open and the other PC has System 2 config open, each with ECS Online mode operating. Each PC has control of the config that is currently opened.
The only difference in using on-line mode is that the user with read-only access to the configuration cannot upload the live changes to the configuration.
The ECS v5.1 manual is quite good in how to set-up the server operation.