Personally, I think this might be good if rather than just picking a pair of skills, the player can opt to pick a situation it applies to in place of one of those skills. And in that situation, the bonus then stacks.
Imagine a martial artist with a Legendary Trait feat that applies to his Drunken Boxing skill, but rather than pick a second skill, he decides he wants to apply it when he's drunk. Now, not only is he a superior martial artist when he's sober, but when he's drunk, stack the bonus and he's now truly on the road to writing his own "Legend of the Drunken Master".