Not sure of how much detail to put so I’ll just put the basics and hopefully not insult anybody. The cluster is, in principle, a standard MS 2 node SQL cluster. So, the process is
Configure the 2 servers to have access to common storage for the required number of disks, we have 4, Quorum, Data, Logs & Backups,
Install the Fail Over Cluster role in both servers and then create a cluster. There is no need to create a role in the cluster
Install MS SQL on both servers making sure to select the cluster installation option on the first server and then the Add node to cluster option on the 2nd server. The actual install is not that much different from a standard install.
Once the cluster is configured you access it over the network in just the same way any other MS SQL server so the connection in Openfire is just the same as for any other MS SQL server, if you want, I can post the string.
From memory I think I used the guid (link below) written by Starwind Software on setting up a 2 node SQL cluster when I set up the my first one. The guide is getting on a bit now as it’s for 2012 but it gives all the basic information needed. You just need to understand the difference with the versions of OS and SQL you are going to uses and what you shared storage is.
Starwind 2 node SQL Cluster guide
So the back end is an SQL Cluster with a 2 node Openfire cluster and in front of that we have a loadbalabcer.org Enterprise 1G to spread the load between the 2 Openfire servers.
Hope this is enough detail