His reaction whenever he feels he’s “switched” is the entire reason why aspects of him are fractured in the first place. The entire goal should be to actively embrace all sides of him so that they don’t have to be conflicting parts of the whole. When he treats one like a parasite and the other as his loving “hyung” when they’re both part of the same body and both love him, all it’s going to do is cause a further divide between them when they should be meshing together. Humans are complex creatures with countless pieces that make us “us”, so the only thing he should be doing is reassuring him that he can love and accept and appreciate every aspect of him so that he no longer feels the need to hide/fight the different aspects of his personality. It’s all him. He just needs to feel safe enough to allow all of him to come through all at once.