Hmm, Sora had to retake the test because he failed. For an introduction, it would make sense if the way the two of them were introduced was, The Meta-World was introduced as a new world in the world of dreams, Sora was lured there during his training in Traverse Town by either Bern or Lambdadelta, Yen Sid sensed something wrong, and then Riku had to dive in after him. Since it's on a higher plain of existence, after all.