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Completing the FAFSA
If your parents are separated or divorced, the custodial parent is responsible for filling out the Free Application for Federal Student Aid (FAFSA). The custodial parent for federal student aid purposes is the parent with whom you lived the most during the past 12 months.
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If you did not live with one parent more than the other, the parent who provided you with the most financial support during the past twelve months should fill out the FAFSA. This is probably the parent who claimed you as a dependent on their tax return. If you have not received any support from either parent during the past 12 months, use the most recent calendar year for which you received some support from a parent. These rules are based on section 475(f)(1) of the Higher Education Act of 1965 (20 USC 1087oo(f)(1)).
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However, in some cases a tie-breaker is needed, such as when the divorce was recent or when there are an even number of days in the year (e.g., a leap year). In such circumstances it is based on whichever parent provided more support. If that is not definitive, then the financial aid administrator at the college will make the decision, and this will usually be based on whichever parent has the greater income.