二 two
两(兩) two, both, a few, pair <- measure word
双(雙) two (as a pair), both, twins
贰(貳) two

Example:

我有兩本書 <- you should use a measure word here

二 two
两(兩) two, both, a few, pair <- measure word
双(雙) two (as a pair), both, twins
贰(貳) two

Example:

我有兩本書 <- you should use a measure word (两) here