Tax Law, § 632(f)
If a husband and wife determine their Federal income tax on a joint Federal income tax return but determine their New York State income taxes separately on one form, their New York adjusted gross incomes are determined separately, in accordance with the rules prescribed by section 112.6 of this Title, that is, as if they had filed separate Federal income tax returns and had determined their Federal adjusted gross incomes separately.
N.Y. Comp. Codes R. & Regs. Tit. 20 § 132.26