Hi
I believe this is mentioned in some of the docs, e.g. here
http://kb.juniper.net/library/CUSTOMERSERVICE/GLOBAL_JTAC/NT260/SRX_HA_Deployment_Guide.pdf
Note: When using multiple clusters that are connected to the same L2 domain, a unique cluster-id needs to be used for each cluster. Otherwise you may get duplicate mac addresses on the network, because the cluster-id is used to form the virtual interface mac addresses.
Not necessarily the most obvious place to look in, but KB articles seem to refer to this doc consistently.