You won't adjust the values of your W-2. If you decided to split the first 6 months of income, that is something you do privately under the supervision of a lawyer.
You will file your taxes as you always do without manipulating any of the information on your W-2. After you receive your returns, you may then decide how you would like to split it.
You can visit the link below for more information on how to file as a married couple divorcing:
https://www.irs.gov/publications/p504/
