Cisco recommends that you do not use the management interface for failover, especially for stateful failover in which the security appliance constantly sends the connection information from one security appliance to the other. The interface for failover must be at least of the same capacity as the interfaces that pass regular traffic, and while the interfaces on the ASA 5540 are gigabit, the management interface is FastEthernet only. The management interface is designed for management traffic only and is specified as management0/0. However, you can use the management-only command in order to configure any interface to be a management-only interface. Also, for Management 0/0, you can disable management-only mode so the interface can pass through traffic just like any other interface.

http://www.cisco.com/en/US/products/hw/vpndevc/ps2030/products_configuration_example09186a00807dac5f.shtml