Name examples where two world champions: ... met in a non-title bout before either had won a world title. ... and later met in another non-title fight when both were ex-champions.
There could be more if I do some research on it. Yesterday I MC'ed a Sports Hall of Fame Induction ceremonies and then I stayed up until 2 AM doing some research on some other things to do with sports, so I didn't get on the computer until almost 1 PM. I never heard that one before, so that was pretty good, because I learned something new.
Yes, you're probably right... if one had the time (and energy!) to thoroughly research this question, more correct answers would probably turn up. But not many, I think! Besides the two you mention, I only have Paul Berlenbach and Jack Delaney... and the funny thing about those two is, that in addition to meeting in non-title fights before and after they were world champions, they of course also fought each other in two world title fights! I could be wrong, but my guess is that Berlenbach and Delaney may be the only two boxers, who have met as both future and former world champions - PLUS in a world title fight. If you can think of another such case, I would be very interested in knowing who that could be.