synchronize domain controllers between accounts

To have DCs in different accounts, you need to have network connectivity between them This could be VPC peering. The following should help

• Walkthrough: Peer with a VPC in another AWS account
• Why can’t I create an Amazon VPC peering connection with a VPC in another AWS account?

Once networking configuration is working, setup your second DC in your second account. You will need to configure your EC2 security group to allow AD network communication between both servers. Your network configuration in the EC2 will need to hard code DNS server to the 2 DCs IPs.