This will never occur in normal gameplay, but a situation has come up in a Level 20 Gestalt game I'm playing in that may also apply to Level 40 Epic games (should such a thing happen).
A player has created a character gestalting Kensai Magus and Inspired Blade Swashbuckler. The end result includes the following two capstones in question.
At 20th level, a kensai gains weapon mastery with his favored weapon, as the fighter class ability.
From Fighter (for reference):
At 20th level, a fighter chooses one weapon, such as the longsword, greataxe, or longbow. Any attacks made with that weapon automatically confirm all critical threats and have their damage multiplier increased by 1 (×2 becomes ×3, for example). In addition, he cannot be disarmed while wielding a weapon of this type.
And from Inspired Blade:
At 20th level, when an inspired blade threatens a critical hit with a rapier, that critical hit is automatically confirmed. Furthermore, the critical threat range increases by 1 (this increase to the critical threat range stacks with the increase from rapier training, to a total threat range of 14–20), and the critical modifier of the weapon increases by 1 (×2 becomes ×3, for example).
According to the hybrid class page:
Each one of the following classes lists two classes that it draws upon to form the basis of its theme. While a character can multiclass with these parent classes, this usually results in redundant abilities. Such abilities don’t stack unless specified.
Thus, the question has come up - because the character is wielding a rapier with a 14-20/x4 crit range that he's then spending around 10 Panache/turn to auto-threaten and other abilities allow him to auto-confirm - do these two abilities stack?