I have a NPM server and an additional poller engine.
If I understand correctly, I must have two FOE engines to protect both engines.
Is my assumption correct?
Correct if you want to protect both.
You will need a Failover Engine for Additional Poller and a Failover Engine for Orion. The tier you will need depends on how many primary products you have. Primary meaning NPM, APM and NCM on the same server.