On a 2 node Windows Server Failover cluster configuration, how do I restart the SQL Server services without failing over to the other node?
Use the Failover Cluster Manager application to manually stop the services.
This tells the cluster you are intentionally shutting down the service, so it won't attempt to bring it back up on another node.
The Failover Cluster service will wait until you manually bring the service back online using Failover Cluster Manager, where you can decide which node to use to bring the service up.
You could stop the passive node, drop it out of the cluster, unplug its LAN cable or otherwise disable the passive node before restarting the active services.
The consequences could be .. um .. "interesting", depending on how cleanly this process is handled and how skilled the tech team that configured the cluster were.