Must've been separate incidents, since Seymour's more than ten years old...
Wonder how and why Sin crossed over into the 'dream world', though. Perhaps because Yevon got a little homesick, and wanted to see his old land, even if it was just an illusion.

Of course, once Jecht became Sin, it makes sense for him to go to the imagianry Zanarkand. attacking it would allow him to extract Tidus and Auron, and it'd give Tidus a reason to want to fight.