Unless I missed it, there is no indication that Joint filers can use $600

The CARES Act seems a bit murky in this regard and just said $300.  From what I remember, I think I read that the congressional notes ("blue book") says $300 on a Joint tax return.  So unless the IRS clarifies it, it is limited to $300 per tax return.

Many charitable organizations are claiming that $600 is deductible for joint return because they want larger contributions.  But as far as I know, there is no indication from Congress or the IRS that $600 is allowed.