If you have a remote-client configuration in which you are using two or more ASAs connected on the same network to handle remote sessions, you can configure those devices to share their session load. This feature is called load balancing. Load balancing directs session traffic to the least loaded device, thus distributing the load among all devices. It makes efficient use of system resources and provides high availability.
To implement load balancing, group together logically two or more devices on the same subnet into a virtual cluster
http://www.cisco.com/en/US/docs/security/asa/asa72/vpn3000_upgrade/upgrade/guide/mitraf.html