It works out of the box.

If a client associated with one controller associates to an AP on a different controller, the new controller will set up a tunnel to the original controller and pass all of the client's traffic through that tunner. The client keeps his same IP address.

The only configuration you have to do to make this work is make sure that both controllers are in the same mobility group.

Mobility Anchor is not necessary for this functionality. By default, a user's "home" controller is the first controller he associated to when he started his wireless session. If you want to force a certain group of clients to always use a specific controller as their "home" controller, regardless of where they first associated, you would use the Mobility Anchor feature for this. Usually, it's used to anchor guest traffic to a DMZ controller.

For more info, see the Mobility Groups FAQ here:

http://www.cisco.com/application/pdf/paws/107188/mobility_groups_faq.pdf