Just fill it in per person. The amounts paid were per person. For instance, if they got the full EIP, 1 and 2, that was still computed per person. If they have one child, and MFS, then whoever has the child with eligibility in 2020 should also have the Advanced Payment amount on that tax return, to show there was a payout for that child.

Thank you but they only got partial filing joint due to phase out. If I split it up one will get a decent rebate credit mostly due to her claiming both kids in 2020.

The advance refund is split 50/50 if it was paid out based on a joint return.  See IRC 6428(e)(2):

In the case of a refund or credit made or allowed under subsection (f) with respect to a joint return, half of such refund or credit shall be treated as having been made or allowed to each individual filing such return.

"per person" means "split in halves."

"but they only got partial filing joint due to phase out."

But that is based on the Prior Tax return. You are filing now, for 2020. The Advanced Payments are split. The eligibility is based on 2020 filing. This is reconciliation.