Cisco PIX Firewall Hardware Installation Guide, Version 6.3
http://www.cisco.com/en/US/products/hw/vpndevc/ps2030/products_installation_guide_chapter09186a008017279b.html#wp1048874
Installing Failover
To install a failover connection, perform the following steps:
Step 1 Power off both the primary and secondary units.
Note Both PIX security appliances must have the same model number, have at least as much RAM, have the same Flash memory size, and be running the same software version. You can connect the PIX 515 to the PIX 515, but you cannot connect the PIX 515 to the PIX 515E or vice versa. Both units must be identical.