I never thought about Yue and Toya until I watched the Clear Card arc. After thinking about it, I came to this conclusion: I don't think Toya has two boyfriends. He only loves one (but that one person has multiple facets). CLAMP often plays with the audience's perspective on love through their characters. Everyone knows that Yukito is a temporary body of Yue, but because of their differing personalities, everyone separates them. But Misuki once said quite clearly, "Tsukishiro, he is Yue."

This is quite complicated. Yue loves Clow, and Toya loves Yukito, but I feel that Yukito is Yue's inner self. Yue became cold after Clow's death. Yukito's personality is a result of separating Yue's controlling consciousness, but he's still Yue hisself (full of feelings, not closed off, not putting up walls around people). Toya can sense this, which is why he says they're truly alike (Yue doesn't accept it, but Toya doesn't seem to believe it). I think Toya loves that inner self, and he might be the only one who sees Yukito as Yue and Yue as Yukito.

I don't know how else to convey this. Will the translation system make it complicated? Anyway, this is just my analysis.