From Answers to Frequently Asked Questions for Registered Domestic Partners and Individuals in Civil Unions on irs.gov:

Registered domestic partners should report wages, other income items, and deductions according to the instructions to Form 1040, U.S. Individual Income Tax Return, and related schedules, and Form 8958, Allocation of Tax Amounts Between Certain Individuals in Community Property States. Form 8958 is used to determine the allocation of tax amounts between registered domestic partners. Each partner must complete and attach Form 8958 to his or her Form 1040.