Route53 zone hosted on two accounts

In this example, "mycompany.com" is a domain whose delegation has been sent to their "A" account hosted zone (the four R53 name servers assigned to that zone have been entered in the domain configuration). The hosted zone in "B" is not being referenced from anywhere, thus it is just existing out in the aether and not being used.

Your customer should decide on a specific subdomain for their "B" records to fall under (e.g. "dev.mycompany.com") and make a new hosted zone with that name. They would then take the four R53 name servers assigned to that zone and enter a delegation (NS record) in the parent zone on "A" with "dev.mycompany.com" as the name and the "B" name servers as the value.

See: "To create a new hosted zone and delegation set for your subdomain" https://aws.amazon.com/premiumsupport/knowledge-center/create-subdomain-route-53/