Two different scenarios. One scenario is to have a DR cluster. 2 independant clusters that are one way synced. As of right now, a custom CDC solution (custom code + Kafka) is needed to achieve this.
Another is because of latency and the requirement for data to be stored in 2 different data center, 2 indepentant mongoDB clusters that are two way synced. As of right now, a custom CDC solution (custom code + Kafka) is needed to achieve this.