HI,
Please find the below points
1)Which is ASA Model which u are using, (becoz the ASA-5510 with out VPN Plus will not allow you to use the management interface for non management.)
2)If The interface on the firewall are set to auto (speed and duplex)it can pass the traffic as the interface are media independent.
or
3) Please create a separate vlan in switch for(failover) and connect the both primary and secondary units.