If a Irrevocable POA was done for two people and only one of those agents in fact was revoked, is that agent in fact that was not revoked still legit

That depends on whether the POA required them to act together or allowed them to act separately.  Whatever the document may say, there is no such thing as an "irrevocable" POA.  The person who granted it can always revoke it as, in this instance, he or she appears to have done with regard to one agent.