There are two clusters of docker swarm. In cluster1 there is one manager and two workers. In cluster2 there is one manager and one worker.
How to join these two docker swarm clusters (cluster1 and cluster2)?
No, not directly. But, you could remove from on cluster and then join the other one. Because, the Manager node handles the worker nodes, their encryption, etc. which can not be done by multiple swarm managers at the same time.
You can always let workers and managers join the swarm node cluster(even nodes are on overlay network). So, in your case cluster1 and cluster2 is running. You have to leave the nodes from the cluster2 so that they can join the cluster1 i.e. a node(manager or worker) can be a part of a single cluster at a time. You must have to docker node rm from cluster2 and then docker swarm join with the token of cluster1 or vice-versa.