Create an organization from the master account. Send invitations to the secondary accounts from the master account. Accept the invitations and create an OU.

Create an organization from the master account. Send a join request to the master account from each secondary account. Accept the requests and create an OU.

Create a VPC peering connection between the master account and the secondary accounts. Accept the request for the VPC peering connection.

Create a service control policy (SCP) that enables full EC2 access, and attach the policy to the OU.

Create a full EC2 access policy and map the policy to a role in each account. Trust every other account to assume the role.