For me, failover in pix was very simple. 2 devices either in same rack or alternate racks with failover cable connecting the 2. But Recently, I had to struggle a lot on client requirement. It was configuring failover for devices in separate buildings.That was the challenge. Here's wat i did:
Upgrade the PIX to IOS ver 7. Use LAN based failover. This overcomes the distance limitation of serial failover cable. With IOS 7 u can either use straignt or crossover cables to connect the devices directly. or connect them to LAN via switch. Hope you'll find out the commands. Revert for anymore help.