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/
Route53 zone hosted on two accountsAccepted Answerasked 5 years ago
Route 53 Hosted Zone Not Forwardingasked a year ago
Three of my hosted zones have disappeared from Route53asked 9 months ago
Lightsail - Private hosted zone - subdomain Route53asked 2 months ago
Hosted zone not configured properly for workmailasked a year ago
How can I use the same domain name in different AWS accounts?asked 3 years ago
DNS not propagating after domain transfer between two AWS accountasked 2 months ago
How to allign the Name Servers of Route53 Public Hosted Zone and Registered Domains in CDK?asked 10 months ago
Route 53 - configuring Private Zone association and Route53 resolver to resolve private zones accross multiple accountsasked 10 months ago
AWS CDK make Route53 records in another accountasked 7 months ago