Under normal situations, if you file jointly, the DCFSA benefit for both spouses is combined on line 12 of form 2441. So under the old limits, each spouse could contribute $2500 (for example). If both spouses contributed $5000, then form 2441 would show $10,000 on line 15. But since the maximum qualified expenses on line 21 is $5000, if the spouses did have more than that withheld in DCFSA benefits, the excess would be added back to their taxable income.
Since the Recovery Plan Act did not make substantial changes to the program except raise the limit, it would seem likely that if two spouses each had a $5000 DCFSA, they would both get the benefit, since the maximum excludable care cost on line 21 should be $10,500 for 2021. However, until the IRS actually releases the forms, there's no way to be 100% sure.
There would be no harm in doubling the DCFSA account, because if it were not allowed, the money would simply become taxable (which it would have been anyway) and there is no extra penalty assessed.
