A two-node cluster is an exception to the rule that a cluster is functional only when a minimum of (n/2 +1) nodes, where n is the number of cluster nodes, are able to serve traffic. If that formula were applied to a two-node cluster, the cluster would fail if one node went down (n/2 +1=2).
A two-node cluster is functional even if only one node is able to serve traffic.
Creating a two node cluster is the same as creating any other cluster. You must add one node as the configuration coordinator and the other node as the other cluster node.
Note: Incremental configuration synchronization is not supported in a two-node cluster. Only full synchronization is supported.