For both credits the requirement to use 2019 Earned Income rather than 2020 Earned Income was that the income for 2019 was more than 2020, therefore it would be logical that the income for the same year would be used for both credits.
According to the IRS
“You may elect to use your 2019 earned income to figure your EIC if your 2019 earned income is more than your 2020 earned income. For details, see Pub. 596.”
“You may elect to use your 2019 earned income to figure your additional child tax credit if your 2019 earned income is more than your 2020 earned income. For details, see the Instructions for Schedule 8812.”
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