I know Arthur Abraham has brought in Sven Ottke as a sparring partner a few times (Abraham started off as an Ottke sparring partner, and Wegner being impressed with him was what led to him getting signed by Universum, so I guess that was returning the favor). Lucian Bute and Glen Johnson were each other's main sparring partners when Bute was facing Andrade and Johnson was facing Dawson. Obviously Hoya-Mosley has already been mentioned. Back in the day of Stillmans, all the top guys used to spar each other all the time. They'd charge admission at the door and you could come in and watch the sparring sessions between guys like Ray Robinson, Willie Pep, Sandy Saddler, Beau Jack, Jimmy Bivins, Rocky Graziano, Bobo Olsen, etc. etc. A lot of these other ones I don't really count because they were sparring partners before they got famous (like Holmes-Ali) or just a little sparring here and there (like Pac-Khan).
Neither guy was particularly famous yet. I imagine it was, since they were Roach's two top guys back then.
Zack Padilla was champ when he use to spar with Mosley(also champ at the time) they use to go at it pretty even and have wars, Unfortunatley Padillas career ended from the brutal sparring sessions(brain hemorrhage). Also Genaro Hernadez and Shane use to spar all the time.