Rinoa and Ultimecia did not know of their past and future of them both, not much can be told, but it should be stated that if Rinoa and Ultimecia would be the same person, they would actually remember themselves, wether it is through looks, voices or memories, it is not stated in the game that they do feel a resamblance to each other.

while under hypnotism of Ultimecia, Rinoa would have been killed by her two times. Meaning, if Ultimecia were to kill Rinoa she would have destroyed herself in the future. It did not happen, but if Ultimecia would go to the past she must be aware of being Rinoa and wouldn't she stay out of sight, afraid she might hurt herself in the future and dissappear.

Unlikely thought Ultimecia does not seem to think about taking actions in own hand and simply tries to sacrifice Rinoa, not caring who she was or what she does. This matters seems handy that Ultimecia and Rinoa are not linked in past, present and future and the by killing Rinoa it would not have hurted herself, else she would not have tried.

The first time Ultimecia tried sacrificing Rinoa as with the help of brining two statues to life, later one she left Rinoa alone in space not caring wether she would survive or not. Rinoa and Ultimecia have no remarable asset to one another, no memories, no resamblances, because they are not the same person.