To use Oracle database links with Amazon RDS DB instances between peered VPCs, the two DB instances should have a valid route between them. Verify the valid route between the DB instances by using your VPC routing tables and network access control list (ACL).

The security group of each DB instance must allow ingress to and egress from the other DB instance. The inbound and outbound rules can refer to security groups from the same VPC or a peered VPC.

Look at https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/peer-with-vpc-in-another-account.html to setup the peering between 2 accounts VPCs.

Ultimately, you may want to create 2 EC2 instances, one in every VPC private subnet where you DB reside and test the connection from here.