Tidus and Jecht both became more than a dream when they touched Sin. Sin made them "real", they are real in the sense that Yuna and others are real, but they are also tied to the dream world, as it created them. When the dream world left, so did they, but in being "real" they didn't just vanish, they went to the farplane. That's kind of confusing, bu that's the best I got at the moment.