Living or spending time at higher altitudes will increase your performance via the number of red blood cells you have in your blood. Athletes do this to increase their performance (after they redescended that is), in which case we are speaking of Altitude Training.
The effect originates in the differences in partial pressure of gazes at different altitudes (notably oxygen, read more here). This difference does not start or stop at a certain altitude, but is a gradual change.
People can acclimatize to higher altitudes - it seems fairly obvious that somebody who already lived at a couple of thousand meters above sea level has a 'head start' when it comes to acclimatizing to a trip of, say, 6000m.
That does not mean that somebody who lives at high altitude is guaranteed to be immune/resistant to AMS though. It is basically impossible to tell beforehand who might develop AMS and who will not - even repeated experience at high altitude does not mean you're safe.
--> Basically the only way to proceed safely is to just treat everyone the same, go slowly following the best practices, and, most importantly, keeping an eye open if anyone is developing symptoms.
To answer your other questions:
If not, at what altitude must the second person live for there to be any noticeable difference?
That really depends on your definition of 'noticeable'. This will basically depend on your kind of setup to verify/test for your result. If you are doing a widespread scientific study with lots of fancy equipment you might be able to measure differences which are a lot smaller than if you, say, compare the performance of the soccer team of La Paz (3600m above sea level) against a soccer team from Amsterdam.
If yes, at what altitude does the difference cease to exist?
It does not. Except, well, at certain altitudes people would rather stop to survive, so everything above that will be similar in terms of performance. ;)
If yes, at what lower altitude must the second person live at for the difference cease to exist?
If we compare extremes the results would probably be rather obvious, i.e. people living at 0 and 50m will likely get AMS at about the same (statistical) rate. If we compare people living at 0 and 5100m (La Rinconada, highest settlement acc. to Wikipedia) the former would probably get AMS a lot more often when going to 6000m. But, again, as we are talking about a gradual process there is no sharp cut-off.