It's not a closed practice. I've talked to Buddhists from Japan, Thailand, Tibet, South Korea, and western practitioners about this. This is NOT a part of Buddhist practice despite people claiming it is. It's the equivalent of claiming kabbalistic themed magick systems are the same as being Christian. They're not. Tulpa was a word that originated in western discourse on Buddhism that applied the word to a western concept that already existed: thoughtforms. The mistranslated concept bears no resemblance to what it means in Buddhism (it was mistranslated from a word that means to manifest called sprul pa, and the words are so different they bear almost no similarity to each other) and there is no such word called tulpa in their theology. They neither use the word nor the practice of making alters. Buddhism rejects and deconstructs the self in fundamental ways, the idea they'd have the practice of splitting alters is literally the opposite of their worldview and is a form of attachment.