A service provider runs PIM Sparse-Mode in its core and wants to eliminate the single point of failure introduced by having one static RP. The design team decides to deploy two RPs sharing the same loopback IP address advertised into the IGP, so that sources and receivers automatically use the topologically nearest RP. Which additional mechanism MUST be configured between the two RPs so that a source registered to one RP is known to the other, ensuring receivers behind either RP can build the shared tree correctly?