I agree with those who have suggested that Willow's resurrection magic altered the Slayer line in ways that "normal" resurrections could not. Here's my version of the timeline:
1. Season 1: The Master kills Buffy, but Xander revives her. She was technically dead, but Xander revived her through natural means (i.e. mouth-to-mouth resuscitation). And he was able to revive her because she was not definitively dead, only momentarily lifeless. I'm not enough of a medical expert to know the logic behind the circumstances when someone can revive a "dead" person, but this was one of those circumstances.
2. Season 2: Kendra arrives, signifying that The Powers that Be (or whomever controls the Slayer line) believed that Buffy was "dead" long enough for he/she/them to call the new Slayer. Drusilla kills Kendra.
3. Season 3: Faith arrives, signifying that only the previous Slayer needs to die in order for a new Slayer to be called. In this case, the previous Slayer was Kendra. So, the Slayer line has been Buffy - Kendra - Faith.
4. Season 5: Buffy swan-dives into a mystical portal, and dies.
5. Season 6: Willow revives Buffy through mystical means. No new Slayer appears, because Faith is still alive (and, at the time, in prison). It has already been established that the Slayer line runs through Buffy, but that in order for a new Slayer to be called, Faith would have to die. Buffy nearly dies at the end of the season, and might have died if Willow didn't remove Warren's bullet from her body. However, this near-death isn't the same as the momentary death from Season 1.
6. Season 7: The First attempts to kill all of the Potentials in order to permanently erase the Slayer line. Notably, the First doesn't attempt to kill Faith while she is in jail, because if it did so, a new Slayer would be called. So, Faith is likely an active Slayer. Depending on your point of view, Buffy is the true Slayer, Faith is the true Slayer, or--because of a cosmic mishap/loophole--Buffy and Faith are co-Slayers. If you believe the last option, then yes, Buffy and Faith would both have to die for a new Slayer to be called. But that's only if you believe that--in the overall cosmic scheme--Buffy and Faith are the singular Slayer, equally sharing the Slayer power and technically a singular entity while literally being two individuals. But if you believe that Faith is the most recent slayer in the Buffy - Kendra - Faith timeline, then only Faith would have to die for a new slayer to be called. If you believe that Buffy is the true Slayer, then why would he/she/them have called Kendra, and then, when Kendra died, called Faith? Wouldn't the PTB have realized they made a mistake and not called a Slayer to replace Kendra? Therefore, the PTB decided to not interfere with the anomaly of Buffy (prior Slayer) co-existing with Faith (current Slayer). However, there must be some rule that says that you can't defy nature and revive a Slayer through mystical methods. Willow did this, and the Potentials paid the price. It remains to be seen what will result due to Willow's scythe magic. Are all of the Potentials now "true Slayers", or do they just possess "Slayer power"? If my logic is correct, we won't know that until Faith dies, either through natural or mystical means.