Normally you can install on one system and the rsBES fileSync ( When it runs ) will automatically transfer the files across to the secondary system as long as all files involved are catered for by the include/exclude.
There is nothing, however, stopping you installing on both systems and we would generally recommend the approach of installing this way starting with the secondary system first for two reasons
1. You are not directly affecting a production server on the initial install so if there are issues you can tackle them first.
2. You are not reliant on the rsBES configuration.