If you're looking for manga similar to Beyond the Beyond, you might like these titles. All recommendations are made by Anime-Planet users like you!
Both MAR and Beyond the Beyond are about a boy who gets transported to another world through a portal made by someone from that world. And when they arrive, both meet companions who are taken with them, as well as one that treats them like a servant (Babbo/BelBel).
Unfortunately, Beyond the Beyond is much shorter and slightly less action-packed, but chances are, if you like one, you'll like the other!
Following its destruction many years ago, the world now consists of many disconnected lands that once again face the threat of annihilation. Sora is a young boy who lives on a tropical island with his friends Riku and Kairi. However the night before the trio is due to set sail on an exploration voyage, they encounter monsters known as the Heartless. During his struggle with them, a large Keyblade appears before Sora and he is transported to the faraway Traverse City. There he meets Donald and Goofy, two representatives of the king who are in need of his power as keybearer. Now, Sora travels with his new companions in order to find the keyhole hidden in each world and lock away its heart to protect it from harm, hopefully finding out what happened to his friends along the way. But with other evil forces afoot, will Sora and the others really be able to save everyone and keep all worlds safe?