Idioms are not only difficult to memorize but are also easy to mix up.
Idioms are not only difficult to memorize and are easy to mix up.
Idioms are not only difficult to memorize but are also easy to mix up.
Idioms are not only difficult to memorize and are easy to mix up.
The words "not only ... but also" work together as a paired unit. We cannot use one piece of this, and then substitute something else for the other piece. That's always incorrect.