I figured this out when I learned more about Time Compression and Ultimecia. It fits the basic of that the theory is not completely right on the games story line. First of all the world exist in many dimension, more then people ever can understand or remember. First there is a alter reality, which consist of the same planet and people, just a different kind of story.

Squall himself went to such a reality of his own past, a meeting with Edea and himself reveales a lot. There is a difference between this past and present reality as they actually can see themselves. I've discovered much more about what Time Compression actually is by going more indept in what Edea and Ultimecia actually mean with compression of past, present and time.

Compressing can be seen as a mixture of things, there is only one alter reality, no past, no present and no future. In other words there would be no past Squall, no future Squall, but only one Squall that doesn't even stand close to the present.

Mixing time is not easy and can endanger many and more things to the reality, which of course nobody can either be born, live or die.

When the Time Compression started, both Ultimecia and Rinoa faced each other. If Rinoa and Ultimecia would be the same person and if Time Compression would start...we would not have been able to control or even see Rinoa fighting Ultimecia.

Time Compression only consist of one person, two person of past and future can not be linked in one reality that is compressed with other times. If people actually believe in the theory, how come we see both Rinoa and Ultimecia in a mixed world of Time Compression?

This makes it sure that Rinoa and Ultimecia are not the same person, not even close because they can easily conversate with each other.