In the anime Tsubasa Reservoir Chronicles, the main characters have the same name and look like as the characters of Card Captor Sakura.
How was this why is that? Are they allowed to use to use other characters of other anime?
Cardcaptor Sakura and Tsubasa Reservoir Chronicle are actually part of the same "multiverse", as well as xxxHolic. Clow Reed is a major character in both Cardcaptor Sakura and Tsubasa, and other characters such as Yukito show up in more than one of them.
There are other crossovers between the different worlds, such as this:
Tsubasa Li is the son of Syaoran Li and Sakura Li. Sakura Kinomoto from Cardcaptor Sakura gave her Star Wand to Sakura Li as the price to Yūko for Tsubasa to go to Clow Country. It is also stated that Tsubasa does not give Yūko his true name, (a counterpoint to Watanuki who gives both name and birthday), instead he uses his father's name, Syaoran Li
Also, according to ccs.wikia.com:
Since part of CLAMP's stated intention of xxxHolic was to unite their body of work into a single universe/multiverse, it seems quite likely that Yūko has a more than passing awareness of the events of Cardcaptor Sakura, Tokyo Babylon, and X/1999, and that while she may not have visited Cephiro herself, she is almost certainly aware of its existence. In a special novel released in Japan about the origins of the two Mokonas, it was mentioned that Clow Reed and herself met the original Mokona of Cephiro (Magic Knight Rayearth) thus making it the model for building the two smaller Mokonas. One is black and one is white. The white one has a red gem and black one has blue gem.
Clamp is known for using crossover characters for all of their different mangas. Tsubasa and xxxHolic are the most closely tied, but there are major crossovers--such as Clow Reed--between Tsubasa and Cardcaptor Sakura.
Is it available to use other characters of other animes into your own without having copyright issues?
Of course. As long as you didn't sign over copyright as part of a distribution deal, you OWN the copyright, and can do whatever you want with the characters. Copyright can be deconstructed as "the right to copy", as in... "I have the right to copy these... you don't".
Why does Tsubasa Reservoir Chronicle ha[ve] the same characters [as] Sakura Card Captor?
CLAMP is very fond of re-using characters. The boys from Clamp Campus Detectives show up in X/1999. The main characters from Tokyo Babylon show up there as well. xxxHolic and Tsubasa Chronicles characters cameo'd in each other's series. These are just the obvious ones I've seen.
Admittedly, I don't know of any other case where they simply AUd the main characters of one series into another one.