Is there actually anyone out there that has managed to go spoiler free about Yuki's little multiple personality disorder? If so, and you're one of them, and you're actually reading this now... gomen nasai! But I did warn you in the introduction.
First a little anecdote. The day I figured out, way at the beginning, that Touya and Yuki were an item, I got online to confirm my suspicions.
I did that faster than Sakura can say "Hoee!". What kept me online the rest of the afternoon was that I just had to click that link that said "Yukito spoilers"... So anyway, my point is that as a result I always knew about Yue (kinda like Touya! ^^;). I spent the better part of the Clow Cards arc anticipating Yue's entrance. Make no mistake about it: I love Yuki, he's a doll. But without Yue he'd be little more than sugary sweet eye candy. Yue is what gives Yukito depth, it's what makes him/them/whatever into one of the most interesting and complex charcters I've ever had the pleasure of running across. And when I finally saw Yue appear... well, let's just say I thought I'd died and gone to heaven.
Done gushing now. ^.^
Now for the heart of the matter. What does it mean that Yue is Yukito's true from? Or perhaps more importantly to the subject matter at hand, what does it mean that Yukito is Yue's false form? And where does one end and the other begin? Are they two sides of the same person, or two almost separate beings sharing the same space (I'd say the same body, but that changes, so I won't)? Are they two separate conciousness, or just the higher and lower planes of the same one?
And what does this mean for Touya??
Surprisingly enough (or perhaps not so surprisingly), for Touya it means... absolutely nothing. It doesn't make one whit of difference to him. Knowing that, objectively speaking, Yukito's just Yue's alter ego, his cover, so to speak, makes him no less real to Touya. The one thing that matters to Touya is that he loves Yukito. He knew from the very beginning that Yuki wasn't what he seemed, that he wasn't human... and Touya fell for him anyway. Perhaps, it was precisely because he was aware that Yukito was even more different than he himself was, that Touya was able to open his heart completely to him. What's not exactly clear is whether he loved just the Yukito side.
Personally I think that even if at first he saw Yue as an extension of Yukito or even as someone else entirely; eventually, he might come to see them as two sides of the same being. That is, if he doesn't already sort of think of them that way... he does tell Yue at a certain point that he's really similar to Yukito. If he doesn't already, I think that in time Touya will come to love the Yue side in its own right. That is, provided that Yue and Yukito can accept that they are the same person. For Yukito this means, in a very real way, a complete revision of his whole sense of self, really not very easy, but a piece of cake compared to the problems that Yue might have with it.
First of all, Yue was desperatelly, head-over-heels, in love with Clow. He never wanted anyone else to be his master, to take his place. He didn't want Sakura to take Clow's place, and it takes him a long time to accept both her and the kindness she shows him. I think it's safe to assume that he didn't want anyone to take Clow's place in his heart either, so he probably resisted sharing Yukito's emotions.
Just how much of Yukito does Yue share? Is it all like a dream of someone else's life? Or are Yuki's emotions his as well? Yue was obviously aware of Yukito's feelings for Touya, but to what extent did he share in those feelings? Was he helplessly watching Yukito fall in love, knowing he would inevitable love Touya as well? Knowing Yue, he would probably view this as a betrayal.
The thing that Yue would have to realize is that Clow wouldn't really want him to wallow in angst and self-pity the rest of his existence. That it's okay to move on and find someone new.