What are the qualifications for fileing income tax jointly?

Only that you are married on Dec 31, 2024.  

You have to be legally married as of 12/31/2024 for a 2024 tax return.

Or if you live in a community property state you have to have been presenting yourselves as married to each other.

IRS Publication 17, page 17 states:

You can choose married filing jointly as your filing status if you are considered married and both you and your spouse agree to file a joint return. On a joint return, you and your spouse report your combined income and deduct your combined allowable expenses. You can file a joint return even if one of you had no income or deductions.

IRS Publication 17, page 22 states:

You are considered married for the whole year if, on the last day of your tax year, you and your spouse meet any one of the following tests. 

• You are married and living together.
• You are living together in a common law marriage recognized in the state where you now live or in the state where the common law marriage began. 
• You are married and living apart, but not legally separated under a decree of divorce or separate maintenance. 
• You are separated under an interlocutory (not final) decree of divorce.

See this TurboTax Help.