No, you cannot configure the same custom domain on two different FusionAuth Cloud deployments at the same time.
The recommended migration sequence is:
Remove the custom domain from the production deployment Configure the custom domain on the dev deployment Configure a CNAME record pointing your custom domain directly to the dev deployment's durable hostname Validate the certificate for the domain on the dev deployment Important ConsiderationsDNS Propagation Time
DNS changes take time to propagate Expect up to 30 minutes for DNS servers to recognize the changes Propagation may take longer depending on your DNS TTL settings Plan for this downtime window during your migrationCertificate Validation
Certificate validation uses DNS-based methods You must complete the DNS configuration before certificate validation can succeedTo minimize downtime, prepare all configurations on the dev deployment beforehand, then execute the domain removal, DNS update, and certificate validation in quick succession.