I think if Rinoa and Selphie were the same character, there'd be no point in introducing Selphie to the story. I think both characters share similar goals but are different in personality. I think Selphie was just mentioning how she and Rinoa share similar goals and possibly certain personality traits in efforts to show others that read her page/blog that Rinoa is just like any other girl and not to weary of her because she's a sorceress (which is one of Rinoa's biggest fears).