Only option is to leave it blank. Your choices are [one or all] on that single agent
Now, i specifically said 'single agent'. What you can do is install a second agent on the target machine, change the listening port to a different unused port, then you can set the server setting in the configuration file that way and have 2 agents listening on 2 different ports connecting to 2 different CCA servers all discovering the same things (assuming the blueprints\mgmt profiles are the same)