I remember for a time Alex Trebek was hosting Jeopardy, but also he was hosting
Classic Concentration on NBC. But then they had him hosting To Tell the
Truth when Lynn Swann didn't work out. had that happened before up till that point?

Bob Barker did both Price is Right on CBS and Truth or Consequences in syndication for a couple of years. Bob Eubanks did Card Sharks on CBS and Newlywed Game in syndication for several years.

The big thing was that the hosts wouldn't host multiple shows in syndication that could compete against each other. Two of Trebek's three shows were on NBC, and the third was in afternoon/evening syndication, so he wasn't up against himself.