Both of them,but most of Chinese usually say 两斤.

两种都可以，只是语言习惯的问题

It depends on the context. 两斤 is used when you are specifying the number of kilos you wished to purchase etc. For example, I wish to get 2 kilos of beef. 我要两斤牛肉. If the beef is packed as 2 kilos, then if you wished to purchase two packages of the two-kilo ones rather than some other quantity, you say, 兩个二斤的牛肉. However, there are certain exceptions. For example, two persons vs two people.... You can say 二人 vs 兩个人. In some cases, 二人 is poetic. For example, two persons viewing the flowers vs two flowers viewer 二人賞花 vs 兩个人去賞花.

两斤 is right。 when the 二 is before measure word ， need using 两。 for example ： 两斤苹果 ， 两个人， 两本书 ， 两百 when the 二 with numeral together , still read and write 二 for example： 十二斤苹果， 十二个人， 十二本书， 两百二十元

^-^ Both of "二斤“ and "两斤“ are grammatically correct. But "两斤“ is the most commonly used. "二斤“ sounds odd and we seldom say that. ^-^ More examples are: 两室一厅 It means two bedrooms and one living room. We don't say "二室一厅“. Quite weird. 一个人，两个人 It means one person, two people. We don't say "一个人，二个人”. Quite Odd. ^-^ Hope it helps.