2. Go to “Central Administration” > “Operations”
3. In the Operations page, go to “Topology and Services” -> “Services on server”
4. In the “Services on Server” page, select the application or web front end server where you want to make the CAS site also available from
5. Then, select the “Custom” option
6. Find the “Central Administration” service and click the “Start” link
7. Go back to the “Operation” page
8. In the “Operation Page”, click the “Alternate access mappings” link
9. Click the “Edit Public URLs” link
10. In the “Alternate Access Mapping Collection” field, choose the “Central Administration application
11. Then, configure the second entry point
12. Browse to the new Central Administration Site from the second CAS server
Additional Step : -
After enabling the CA on the second server I received the following error in the event logs and could not access the CA site from the second box.
Event ID: 1057
Computer: APPDEV
Description: The identity of application pool ‘SharePoint Central Administration v3′ is invalid, so the World Wide Web Publishing Service can not create a worker process to serve the application pool.
Resolution: –
1. Open IIS Manager
2. Open Application Pools
3. Right click SharePoint Central Admin V3
4. Properties 5. Select Identity tab
6. Re-enter MOSS admin account password
7. Select OK
8. Restart SharePoint Central Admin V3 application pool
9. Browse to App server CA site successfully
10. Check Event logs are ok
Update: I have since found the following. When you run the SharePoint Best Practices Analyser it will actually warn you that the CA Site should be configured on multiple servers in the farm for high availability. So I guess this is supported after all 
Title:Highly Available Central Administration Capability
Severity:3
Description:
Only one instance of the SharePoint 3.0 Central Administration Web site exists in your installation. To make sure that you can administer the farm in the case of a failure of a server hosting SharePoint 3.0 Central Administration, you should have SharePoint 3.0 Central Administration installed on at least two servers in the farm.