In SCOM 2012, A Management Server can only run one network device discovery rule. So if you already have one network device discovered, when you try and create a new discovery rule you won’t have any Management Servers available.
You’ll either need to:
- edit the original discovery rule to cover both network devices
- add another Management Server
http://technet.microsoft.com/en-us/library/hh278846.aspx
“Each management server or gateway server can run only one discovery rule. You specify a single management server or gateway server to run the discovery rule and a management server resource pool to perform the actual monitoring of the network devices.”
Kevin Holman does a great walk-through here.