其实都可以，但两条腿更常用。

你可以说一双美腿。两条美腿，一双大长腿，两条大长腿。如果没有中间这个美字或者大长来补充，别人抓不到你说这个短语的意图

ha. it is a very good question. I have never thought of why we do not say 一双腿， I don't think I ever used that or heard others said so even though it is grammatically correct. 我们可以说一双手，但是不说一双腿。估计就是一种习惯。 If we heard people say一双腿，we would understand what it means but we know that is not from a Chinese person.

Both are ok and equally common, best of luck with your Chinese learning.

这是量词的固定用法