I disagree that xxxHOLIC and TRC crossover less as the series progress. If anything they've been crossing over much more!
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The soul harvesting being explained. Sakura retrieving the little birdy egg as Yuko's payment. Real Syaoran's mysterious reason behind helping to save Watanuki, which will no doubt become very relivent later on...
Yuko talking about Fei Wang in the latest HOLIC chapter made it sound like the two mangas will crossover even more in the future.
The tag line was 'The one who, "in order to make his wish come true, will break everything". Fei Wang Reed... What is his true nature?! And what will embracing that "dream" bring about for Watanuki and the others?!'
Yuko said that both she and Clow knew about Watanuki before he came to her. So it's pretty definate that Wakanuki will have quite a role to play in the climax of the TRC story since it was Yuko and Clow that have been working together to foil Fei Wang for a long time.
So you're very right. You really need to read both manga series to have the full picture of what is going on.
Is TRC and xxxholic the same story?
not really. their 2 diferent story lines and genres, but they both are relevant to eachother. Yes you CAN read TRC without xxxholic, but youll get much more incite if you read holic along side, and if you read xxxholic without trc, youll just be left confused as to who FWR exactly is and whats up with the 2nd syaoran. Show content
and NOW youll be wondering "whats this dream yuuko's talking about?" etc