How to migrate 2 different Cassandra databases into one cluster

We have two separate Cassandra clusters, each with six nodes: Cluster A and Cluster B. Our requirement is to merge these two clusters to save costs. Because this is a critical application, we need to plan this with no downtime or a very minimal cutover window. We can create tables from Cluster B to Cluster A and copy data using snapshots, but how can we ensure continuous writes for live data into the new Cluster A and minimize downtime? Any suggestions on how to proceed with this activity?