To put it simply:
Tidus and Jecht are a dream of the Fayth. Tidus is a replica of Shuyin in part, in that he took on Shuyin's physical characteristics, but not his experiences.
Shuyin, the person, died a long time ago. He got shot.
Shuyin, the aberration that appears in FFX-2, is a collection of the memories/emotions/vengeance collected at the time of the real Shuyin's death. In every way, he is a Fiend. A superpowered Fiend with a girlfriend. In this case, he is the metaphorical shadow of his former self, created by the Pyreflies.
Do you know why people use the metaphor of a shadow? Because it sounds cool. When you stand in front of a light source, you create a two dimensional projection onto the ground, based on how the light hit you. But when you reach for a shadow, you can't touch it, and you have lost a spatial dimension (depth) when observed. In fact, you can even screw with the shadow by changing the light's angle, or by placing your hand in front of the light source. What people are trying to say, is that there is no substance to the Shuyin in FFX-2; he's just a collection of memories of being pissed. Okay, so he can try to blow up the world, but what he represents is not wholly what the real Shuyin represented. He has lost an element of himself; he's not particularly deep or insightful. He's just there. How else can you describe a shadow as anything other than "it just happens to be there?"