Houkago Saitensei!

A magical circle appeared above Earth ten years ago, linking it to the parallel world of Alterna. This event led to the creation of the Alternaphone, a device enabling people to reincarnate freely between the two worlds, allowing Earth residents to become adventurers and Alterna inhabitants to live as ordinary citizens. The story centers on two girls who have lost their memories—Utahou Kurami, from Earth, and Melua Solaris, from Alterna—as they travel between their everyday lives on Earth and their adventures in Alterna.