If you live in a community property state, you will generally split it all 50/50. Some states have specific rules regarding the split, but most community property states follow the 50/50 rule. The exception would be income earned on assets or investments that were owned by a spouse prior to the marriage and not commingled. Also, inherited income that was not commingled.
For Example; You earned $100,000 of income and your husband has $60,000. Jointly you have $160,000 and in a Community Property State you each have $80,000.00. So in this case you will have subtracted $20,000 to your income and your husband would have added $20,000 to his.
Community Property states are: Arizona, California, Idaho, Louisiana, Nevada, New Mexico, Texas, Washington, and Wisconsin.
FYI - When you file as Married Filing Separately, if one spouse itemizes on Schedule A, the other must also itemize on Schedule A.