You should put zero for John (the husband).

John is claiming the child for TY 2021 and is filing on a separate basis.  Accordingly, only he is entitled to the $3,600 Child Tax Credit.  Because he did not receive any advance payments during the calendar year, he will receive the full credit when he files his 2021 tax returns.

Jane (the mother) received advance CTC payments for TY 2021 even though she is not claiming a child for this tax year.  Accordingly, she will need to report the $1,800 payment on her 2021 returns and her refund will be reduced and/or her balance due will increase by this $1,800.