I think that Jecht and Tidus 'existed' only as a dream -- in the fictional (prosperous) Zanarkand of 1000 years ago -- created by Yu Yevon. Somehow through the interaction with the then-current Sin, it brought Jecht to the real world of Spira. In a similar manner, Jecht wanted his son to free him from being Sin, and hence brought Tidus to Spira.