Federer currently has 16 slam titles, Nadal has 8. The second highest slams won at the moment is Sampras with 14. Let's take situation wherein, when everything is said and done, Nadal surpasses Sampras in the total slam column (or equals him). Now if we have two players from the same era and if they end up being one and two in the all time total slam titles won column (including all of the other great achievements that they manage to achieve till they play the sport like weeks at number one, ms events won, davis cup, etc. and so on)
If such a thing happened, would you think it would be fair to call this the greatest era with two all time greatest players?
Without a doubt, they would have surpassed the Sampras, Courier, Agassi, Ivanišević, Becker era.
And I think when its all said and done, Murray will have probably won a slam or two, maybe Nole one or two more, possibly Del Potro(and than we already had players like Roddick, Hewitt, Safin, etc. at the beginning) and who knows who else... So the Federer-Nadal era is turning out to be the golden age of tennis, its rewriting a lot of what was considered to be tennis history.
And if it ends with the number one and two of all time well than there is no questions asked this will be a new peak for tennis.