Bumping this because I have the same question, and I imagine a lot of people are going to be in this scenario.
Looking at form 2441 from 2020 and thinking about how it will look in 2021, is the IRS just going to change "$5000" on line 21 to "$10,500"? If so, it seems like you could do what you suggest and get to exclude the full $10,000 from income, but it's not clear to me.
And the follow-up question is, let's say you do what you suggest here and each elect $5000--what's the downside if you guess wrong? You just wouldn't be able to exclude that extra $5000 from income and are taxed on it (line 26 from 2441)?
Interested in other thoughts, as this has big impacts for families trying to muddle through open enrollment...
