Fantasy World Switch

Karna's family recently moved into the ancestral mansion they inherited from a distant relative. The mansion has a story about a mysterious antique door in the room of the mansion's first owner. At first, it was treated as a local legend, but soon Karna found out that there was more to it. One day the door suddenly opened on its own and someone who looks eerily familiar stepped in through it. The person claimed to be Karna's parallel world twin from a world of magic and he wanted to switch worlds with Karna since he was more interested in the modern world's science.
See Karna's Journey in the World of Magic and Monsters, Elysium.