I did notice that as I saw more interaction between Kurogane and Fai. Although there are some very obvious differences, they do remind me a bit of Touya and Yukito. Touya and Kurogane are similar in the since that they both have rough personalities (Kurogane, of course, much moreso than Touya), somewhat similar physical features, and they're both distant and closed-off emotionally to most people (with only the expection of a few individuals). Yukito and Fai are similar for their kinder, gentler, and easy-going personalities, their much more delicate and ethereal appearances, their apparent enjoyment of teasing their rougher counterparts (Touya/Kurogane), and the occasional hints of a sadness that lies behind their cheerful fronts. Fai and Yukito also seem to share a similar relationship to Sakura.
However, the relationships between Touya and Yukito and Kurogane and Fai are very different, in my opinion. For one, the (romantic) relationship between Touya and Yukito is canon - well, at least in Cardcaptor Sakura. It is likely to apply to Tsubasa, as well, particularly considering that they seem to have a close relationship no matter what world they are in. It kind of implies (at least, to me) that their souls are connected. And they are obviously at ease and affectionate with each other... they have the air of two individuals that have been in love and together for a long time. Kurogane and Fai (in the romantic sense) are not canon, and there is no real indication that they ever will be. However, they do seem to have a chemistry that makes for a potential relationship - and considering that this is CLAMP, an eventual relationship between them is entirely possible. Since they have only recently met, they lack the closeness and comfort that exists between Touya and Yukito, but I wonder if their relationship will gradually become a bit more similar to Touya and Yukito as time progresses. They are very similar in personality, after all. :p
