A cluster must have 2 nodes either in a single site or as a primary + secondary.
If you try to create first the cluster with just one primary site and one node in that side, it will fail to save the configuration and it will come up with the following message
Error: PAM-CMN-5064: Cluster must contain at least two members, including this member.
So the solution is to define the primary site with one node, and then the secondary site with the other node and to save the configuration locally, then define the same at the secondary node (use the shared key from the primary node and load the configuration from the primary site node) and save configuration to cluster.
After this the cluster will meet the requirement to have 2 nodes and it will be possible to save and to start it.