If my significant other and I own a small business, who files for deductions? Both of us or just one of us? We are not married and are not filing together

You may have established a partnership.  A partnership is an unincorporated business owned by two or more persons associated as partners. Often the same persons who own the business also manage the business. Many small retail establishments and professional practices, such as dentists, physicians, attorneys, and many CPA firms, are partnerships.

A partnership return may be required to report the income and expenses (Form 1065}   The net profit/loss shown on your individual tax returns is determined by the partners' ownership percentages.

If tow of you won the business shall we say 50 -50 each or something like that then you folks have a partnership and a schedule K-1 shows how much profit or lose percentage of the schedule C goes to each partner.  Each has a Schedule K-1 to file with.  But a partnership return must be filed to do so. Hope this helps.

You also have the option of forming a Corporation. You could start with a regular Corporation and change it to an S-Corp.