If we see Pacquiao fight Floyd, it's pretty hard to argue that that wouldn't be a case of a p4p #1 & #2 fighting each other. When was the last time that happened?
I am not so sure that these two guys are necessarilly the no 1 and 2 pound for pound guys. They probably are, but to be honest, I think that Wlad, Vitali, Wonjongkam and Possibly even Hopkins after his recent efforts have decent claims as pound for pound equals. (not that i think we should see Wonjongkam try to fight the others to settle the issue). Back to the question though, Leonard vs Hagler is probably the last standout i can think of and even then, i am not sure that this was necessarilly the case either.
I believe it was Chavez-Whitaker in '92. During the Jones-Toney fight Whitaker was #1, Toney was #2 and Jones #3. Jones didn't really take over the top spot from Whitaker till Whitaker's struggle with Rivera in '95-'96.
In terms of flat out ability Donaire is the only other man with an argument and maybe Khan/Gamboa (in terms of ability not achievement) There's Toney-Jones but also Whitaker-DLH was marketed as 'P4P' and Tito-DLH must have been up there, while not being 1 and 2, Hopkins-Tito were both pretty high, some foolishly had Tito no1 before that fight. Pacquaio and Marquez 2 were both top3 P4Pers I think
Chavez/Whitaker were #1 & #2 (after this fight Whitaker was #1 & Chavez #2) DeLaHoya/Whitaker were #2 & #3 (DeLaHoya became #1 after this fight), Jones was #1 Toney/Jones were #2 & #7 (Jones moved up to #2 after this fight)
They were #1 & #3 (after the fight, they were #3 Trinidad and #4 DeLaHoya. Jones was #1 and Mayweather was #2.
Some arguables: Chavez-Taylor Leonard-Hearns Duran-Leonard Harada-Jofre Griffith-Rodriguez Robinson-Gavilan Armstrong-Ross
Roy was better than them all, though there were some during those times that would have argued that the above fighters were better than Roy...
De La Hoya/Trinidad were #1 & #3 (after the fight, they were #3 Trinidad and #4 DeLaHoya. Jones was #1 and Mayweather was #2) Trinidad/Hopkine were #2 & #7 DeLaHoya/Mosley were #2 & #5
SS, when Gavilan fought Ray Robinson in 1948,the Kid was never rated #2 P4P. Ezzard Charles, Ike Williams, Archie Moore,were along with Willie Pep considered better P4P fighters of that time. His reputation grew wiyh time...