Another point of interest is that a high percentage of the double surname characters tend to be very powerful
Examples: Linghu Chong, Dugu Qiubai, Dugu Yihe, Dongfang Bubai, Murong Bo, Murong Fu, Ouyang Feng, Gongsun Zhi ; ; ; ; Shangguan Jinhong, Ximen Chuixue, the Nangong family in the Gu Long universe,
There are fewer such characters with inferior martial arts, and most of them are rather obscure. I can only think of a few (not counting the relatives of the above like Ouyang Ke): Sicong Xuan, the leader of the Divine Agriculture Clan in DGSD; Ximen Guanzhi from Ode to Gallantry (the actual level of this guy is unknown, but he should be pretty good).
Logical
raison d'être: wuxia is set in ancient times, and double surnames are also associated with ancient times. By implementing a bunch of powerful double surnames, it makes us feel closer to the exotic ancient.