1.She can both speak and write Japanese.

2. She can not only speak but also write Japanese.

Sometimes the conjuctions both ....... and , not only ........but also seem similar in usage.

But there may be some differences between two.

Could anybody explain their usage?
The not only...but also correlative conjunctions add a certain frisson of surprise to the second term, stressing it as a relatively unexpected addition. Both...and simply sets up the two terms in parallel.
<Both...and simply sets up the two terms in parallel.>

If there is stress on "and write", it can do the same as not only...but, IMO.