When a student's parents are divorced or separated and not living together, only the custodial parent's information needs to be included on the FAFSA. The custodial parent is the one you lived with most over the last 12 months. If it was equal time, then it's the parent who provided the most financial support during that time.
If the custodial parent has remarried, the stepparent will also need to report their financial information on the FAFSA.