Any interface can be used as the LAN failover interface. We suggest a separate interface because there can be quite a bit of traffic going over it, but if your PIX's are relatively lightly loaded then you should be OK. Monitor it for a while and make sure you're not seeing any unnecessary failovers, these may indicate that the interface is getting too busy and your dropping hello packets. If this starts to become an issue you may need to purchase another interface card for each one and set that up as your failover int.
