There's no strict black and white rule. In general, you split them based on who paid for what. If your incomes were combined, you can split them in any reasonable way, except that if there are person-specific donations then only that person should claim them (his medical expenses, her donated clothes, etc). Just be aware that if one spouse itemizes, the other spouse must also itemize, so you can't shift all the deductions to one spouse and the other spouse use the standard deduction.
