The King James version says :
12 For the word of God is quick, and powerful, and sharper than any two-edged sword, piercing even to the dividing asunder of soul and spirit, and of the joints and marrow, and is a discerner of the thoughts and intents of the heart.
The NWT says:
12 "For the word of God is alive and exerts power and is sharper than any two-edged sword and pierces even the dividing of soul and spirit, and of joints from the marrow, and is able to discern thoughts and intentions of the heart.
The only striking difference is the words alive instead of quick, and the omission of the word asunder. Everything else is pretty much the same albeit written slightly differently.
I'm not sure why we use the word "alive" and the KJV uses "quick", but they both make the point about the impact of the word of God, so the fact that two different words were used doesn't change the context or the meaning.
As for the word "asunder", it is so unusual today that I had never seen it before and had to check the dictionary. Asunder means "apart". Since the sentence already includes the word "dividing", using the word asunder in the same sentence can be redundant and therefore unnecessary. Removing that word simplifies the verse.
Please note that both versions are perfectly fine and I don't think one is better than the other. The NWT uses more contemporary language, but there is nothing wrong with either translation.
Hope this answers your question.
