Hi .. the 'failover' feature as such does not exist on the device itself but in the network infrastructure. The IPS behaves as a bump in the wire and so depending on how or where you place them they can operate together and/or at the same time offer redundancy. For example I have just implemented 2 IDSM-2 modules on a Cat 6513 I have configured them exactly the same and linked together on an etherchannel bundle in that way they are load balancing the inspection of the packets and at the same time providing redundancy in case one of them fails.
In your case perhaps placing the IPSes between layer 2 switches and multitayer 3 devices. With the help of STP running on those switches you should have the redundancy you want.
I hope it helps .. please rat it if it does ..
