New in SP1, Microsoft will now allow two ACS collectors to point to the same ACS database, but with only one active at a time. One acts as the primary collector, the other acts as the failover/secondary collector, which can only be enabled when the primary one failed or is disabled.

Pete Zerger and I have written a step-by-step walkthrough of the configuration and testing process for redundant ACS collectors. You can download the guide here.