Elasticsearch always requires a strict majority of master eligible nodes to be available in order to elect a master, so with only 2 master eligible nodes both have to be avaiable (strict majority of 2 is 2) for the cluster to function properly. This is why 3 master eligible nodes is reconmmended, as the majority of 3 is 2 and one node can go down without affecting the cluster. Have a look at this section in the docs for further details. For small clusters it is common to have 3 nodes that hold data and all are master eligible.