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Why do I get a warning when I have only two controllers/managers in my cluster?

Article ID: KB000528

For Docker Universal Control Plane to be highly available and fault-tolerant, you need to join multiple controller nodes (also known as manager nodes):

Controllers/managers Failures tolerated
1 0
2 0
3 1
5 2
7 3

With only two controller nodes, UCP doesn't tolerate any failures. You should add additional controller nodes as soon as possible.

For high availability you should have 3, 5, or 7 controllers nodes. More than 7 can negatively impact the performance of UCP. You can also join as many worker nodes as you like, to be able to handle more workloads. To join more nodes to UCP, navigate to the 'Nodes' page and click 'Add Node'.

Note: To learn more about UCP High Availability, visit http://docs.docker.com/ucp/high-availability/set-up-high-availability/.