If you go to the Data Collection preference in Remote Desktop and drag a second Remote Desktop Administration computer into the "Remote computer address" field, the field may not display the address correctly. If you try manually typing the Remote computer's IP address in the field, the information disappears when you click Select.
This happens because the Data Collection feature requires two unique software serial numbers, as explained on page 86 of the Remote Desktop 2.2 Administration Guide:
A remote computer that collects the data needs to be a licensed ARD administrator computer...
Check the current administration computer and the targeted remote computer to ensure that each has a serial number entered and that each is unique. To change a Remote Desktop serial number:
Tip: Additionally, a remote data collector must have port 3283 open for both TCP and UDP. TCP port 5900 must be open to do a Control or Observe of the computer, but otherwise doesn’t need to be open. UDP port 5900 would need to be open, if for some reason, you want to do a Share Screen to the computer.