Here is my top four lightweights from 1940s on to present...Tough to beat..I'll start Ike Williams Beau Jack Bob Montgomery Sammy Angott So many more I saw fighting each other frequently...And your tops, fighting at the SAME TIME.....
Yeah, I think you've taken the cake right there. I'll throw out Benny Leonard, Lew Tendler, Freddie Welsh, and Willie Ritchie, just to add to the discussion. Then again they were about 20-30 years prior, and as such wouldn't qualify by your criteria.
Birt's one would be the one i would go for if someone hadn't already. I'll say Lou Ambers, Henry Armstrong, Baby Arizmendi and Barney Ross just bridging the eras.