Claiming advanced child tax credit

Does anyone know what would happen if a taxpayer who claimed a dependent child for 2020 agrees to take the advanced child tax credit starting in July but their ex will be claiming that same child for 2021 ?
I thought I read where the IRS was going to address this but not finding anything for sure yet. I know they shouldn't do it if not claiming this same child on their 2021 taxes but I've been asked the question a few times lately.

Thanks for any help!

Does anyone know what would happen if a taxpayer who claimed a dependent child for 2020 agrees to take the advanced child tax credit starting in July but their ex will be claiming that same child for 2021 ?
I thought I read where the IRS was going to address this but not finding anything for sure yet. I know they shouldn't do it if not claiming this same child on their 2021 taxes but I've been asked the question a few times lately.

Thanks for any help!

Does the taxpayer have to 'agree' in order to take the advanced child tax credit or is that automatic? What is the process if an agreement is needed?