Separate machine entries with a comma and no spaces. The first example is correct, all others are wrong (notice the spaces in the wrong examples):
MonitorRSCMs = orw-rscmserver-dt:9000,147.35.84.166,alh-rscmserver2-dt
MonitorRSCMs = orw-rscmserver-dt:9000, 147.35.84.166,alh-rscmserver2-dt
MonitorRSCMs = orw-rscmserver-dt:9000,147.35.84.166 ,alh-rscmserver2-dt
MonitorRSCMs = orw-rscmserver-dt:9000 , 147.35.84.166 , alh-rscmserver2-dt
If you use hostnames rather than IP addresses as parameters for MonitorRSCMs, be sure that the machine running the iCDB Server Manager can ping the RSCM machine outside the subnet, and that the RSCM machine can ping the machine with the iCDB Server Manager.
You can also add machines that are in the same subnet, however you can access such machines through the iCDB Server Manager GUI with View > Show All Servers. See Broadcasting iCDB Servers.
You must restart the iCDB Server Manager to recognize the changes made in the ServerManager.cfg file.