For state tax purposes, you can use any reasonable method to allocate unearned income from jointly held accounts.
The most obvious approach is to allocate 50% of the income to each joint owner. As joint owners, each has equal access to account assets so it is reasonable to state that each has an equal claim to those assets and any income from those assets.
Another approach would be to allocate the income according to the source of the assets in the accounts. If one spouse contributed all or most of the assets in a given account, then you could reasonably allocate all or most of the income to that spouse for state tax purposes.
