Hi Patrick
Please have a look at the links below
http://www.cisco.com/c/en/us/support/docs/security/asa-5500-x-series-next-generation-firewalls/110740-asafailover-transparent-mode.html
http://linuxsysadminblog.com/2009/02/cisco-asa-5505-activestandby-failover-configuration/
Depending on the version of ASA you are running there are some license changes regarding failover
http://etherealmind.com/cisco-asa-failover-licenses/
Thanks
Pantelis