Two Dark Lords?

[Boromir88]

Inziladun's got it. In Tolkien's letter to Milton Waldman, he talks about the Ring and Sauron's power being in rapport:

While he wore it, his power on earth was actually enhanced. But even if he did not wear it, that power existed and was in 'rapport' with himself: he was not 'diminished.'

So, this explains that even if Sauron lost the Ring (as he did in the Last Alliance), the power would still be bound to Sauron. The only way to break the bond would be to destroy it, or someone else to claim the Ring, and usurp Sauron's place. It was the latter which Sauron feared the most, because if someone became the new 'Ring-lord,' Sauron would be left powerless just as we was after the Ring was destroyed.

I won't quote Letter 246 in full, because it's a rather long chunk, but Tolkien does go through possible Ring-lord candidates. He says even when Frodo claimed the Ring in the Sammath Naur he had nowhere near enough power, he wouldn't even be able to control the Nazgul. The Nazgul would have taken Frodo out of the Sammath Naur and straight to Sauron. Aragorn (who was able to best Sauron through the Palantir) would not have the strength either - as no mortal would have. Aragorn barely beat Sauron through the palantir, because the palantir was rightfully Aragorn's, and the contest took place at a distance. Had Aragorn been closer, say as close as Denethor was in proximity to Sauron (Denethor who also by right could use the palantir, but did not have Aragorn's mental strength - and was geographically closer to Sauron) Aragorn would probably have lost his mind in a contest against Sauron, as Denethor did.

So, that pretty much leaves (in Middle-earth) the Elves and Maiar, like Gandalf and Saruman. Now Tolkien does say that Gandalf may be the only one capable of besting Sauron for mastery of the Ring, because they are of the same order, and a Ring-wielding Gandalf might beat a ringless-wielding Sauron...however this was all speculative. Elrond and Galadriel also thought they could use the Ring to best Sauron, however Tolkien states that they soon rejected this, knowing the Ring's essential deceit was to fill its bearer with delusions of supreme power.

In any case, a battle against Sauron would certainly have to occur, if someone like Gandalf wanted to claim mastery of the Ring. I don't know what contest it would be, I've always imagined something like a battle of 'wills' as was seen between Aragorn and Sauron, Denethor and Sauron, through the Palantir. But who knows?