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

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.