The actual Shuyin is the one who lived 1000 years ago in Real Zanarkand, and was shot and killed by Bevelle soldiers. He becomes an Unsent, and you see Unsent Shuyin in X-2.

There is another Shuyin that was a dream of the Fayth. This Shuyin lived in Dream Zanarkand 1000 years, and "dream died" from unknown causes.

Tidus being his own person that was "based" off Shuyin is somewhat plausible, but I've heard people say that he was the Fayth's interpretation of the real Shuyin, which is false, because there was a dream-Shuyin that was created 1000 years ago and dream-died in DZ.