The latest version of Solr CDCR across collections / clusters is in active-passive format, where we can index into source collection and the updates gets forwarded to the passive one and vice-versa is not supported.
We are try to get a design ready to index in both collections and the updates gets reflected across the collections in real-time (given the backlog of replicating updates to other data center). ClusterACollectionA => ClusterBCollectionB | ClusterBCollectionB => ClusterACollectionA.
The STRONG RECOMMENDED way to keep indexing in ClusterACollectionA which forwards the updates to ClusterBCollectionB. If ClusterACollectionA gets down, we point the indexer and searcher application to ClusterBCollectionB. Once ClusterACollectionA is up, depending on updates count, they will be bootstrapped or forwarded to ClusterACollectionA from ClusterBCollectionB and keep indexing on the ClusterBCollectionB.