Do you have Clients connected to the Server.
When yes and you have client older then 10.1.x be aware of the upgrade.
The clients will stop functioniong when you perform the upgrade and not implment a small change on the server where support can help you with.
Or request SEMS 3.3.0 MP1 HF1 when this is the case.
If you don't have clients older then 10.1.x don't worry about.
You can just upgrade both servers the order doen't matter since there is no master or slave in 3.x.
The cluster will stop replicating as soon one server is upgrading and restart clustering as soon both are on the same version.