This GM would have the guard take the ring
Abe the PC told the truth to Carlos the NPC guard; no rolls need be made to tell the truth. Bill the PC made a Bluff skill check to lie to Carlos the guard, opposed by Carlos's Sense Motive skill check. Bill wins, and Carlos believes Bill's lie. But Abe's truth and Bill's truth are at odds, and Carlos believes them both! Carlos knows that either this is all a hilarious duplicate-ring (or time-travel!) mixup or one of them is lying. This GM supposes that, as a city guard, Carlos will opt to think the latter.
Abe, though, has offered Carlos the guard a truth that can be verified. Carlos should take the ring, investigate its provenance, and either return the ring to the owner if Abe's story proves true (which it will) or return the ring to Bill if proves false (which it won't unless Bill's laid the groundwork for that beforehand). If the ring goes back to the owner, Bill should keep an eye out for Carlos the guard—he'll likely be suspicious of Bill when next they meet.
If for some reason Carlos the guard can't take the ring, his best bet is to make Abe and Bill repeat their stories. Several times. This'll force Bill each time to make another Bluff skill check to lie that's opposed by Carlos's Sense Motive skill check. Eventually—just like in any interrogation scene—, Bill will screw up (rolling low), Carlos will detect the fib (rolling high), and the jig'll be up for Bill (unless, y'know, Bill's lawyer arrives first). Abe'll always be in the clear, though, so long as he consistently tells the truth.
However, it's possible that because of the wide span two characters can have in opposed skills that there's no mathematical way for Carlos to penetrate Bill's ruse. So even after Carlos listens to their tales a couple of times, he can't resolve believing both Abe and Bill: Carlos is out of his depth. Carlos needs to haul these perps to the station, kick this incident upstairs, and have better-skilled interrogators go to work on Abe and Bill. (This GM tends to populate the city guard with at least one high-level commoner with feats and so on sufficient to make the dude ultraspecialized in the Sense Motive skill for exactly this sort of problem.)