I need to apply SP2 for SQL Server 2008 R1 on one instance of a multi-instance cluster. I was going to apply the SP on only one node of the cluster. This is because on the other node of the cluster I cannot get the necessary downtime on the other active instances. Although I am not patching these other instances, normally you have to reboot the server/node when applying a SP and they would be affected by the reboot or by failing them over to the other node of the cluster.
I will apply the SP on the other node in future when I can organise the necessary downtime.