The other Tobi is also known as Guruguru. When Madara summoned the Demonic Statue, it brought forth Tobi too. Madara believed that Tobi was an imperfect clone of Hashirama Senju. Tobi was not a fighter, but he could mold his body around a person like a suit to make them stronger. The other Tobi and Obito are not related per se. But they are comrades.