The Riku Replica and Xion have never been seen interacting and most likely never met yet the two are from exactly the same place and have exactly the same creator. Both were created as replicas by Vexen in Castle Oblivion. The Riku Replica was obviously created to be a replica of Riku while Xion was created as a replica of Roxas. Both already had hearts and received memories that did not belong to them. Xion and the Riku Replica were also both eventually destroyed when they supposedly merged with their original selves.
RepliShi is not that popular of a Kingdom Hearts ship but is does have its fans. It is mainly overshadowed by the Rion which involves the real Riku with Xion instead as the real Riku actually did interact with Xion. However, they are both replicas who were created in Castle Oblivion by Vexen and were never supposed to exist in the first place. Many fans of the ship feel that this gives them something in common and gives them compatibility.