The JCT explanation of Sec. 62(f)(1) in relation to eligible individual, in the form of footnote (#76), is very clear the $300 limit is applicable to a tax-filing unit.  In other words, it is the same whether the return is filed as MFJ or MFS.

Sec. 62(f)(1). The $300 limit applies to the tax-filing unit. Thus, for example, married taxpayers who file a joint return and do not elect to itemize deductions are allowed to deduct up to a total of $300 in qualified charitable contributions on the joint return.