If you are married and filing jointly, your deduction is still limited to $300. The $300 limit is for a "filing unit" (i.e. a return), not per individual. It applies regardless of filing status.
From The new charitable deduction for non-itemizers: An individual eligible to claim the deduction is any individual who does not elect to itemize deductions for 2020 (Sec. 62(f)(1)).
