Connect 2 AWS EC2 Free Tier instances

I'd like to try the AWS free tier program before scaling up if all goes well. I created 2 AWS free tier accounts, each one has an EC2 instance and they are both in the same Availability zone. Now I want to be able to send files thru ftp from one to the other using their private IP addresses in the default VPC 172.31.0.0/16 (hence avoiding to use my outbound bandwidth) but I cannot manage to get it working. I have created an inbound rule on each one to accept all traffic from the other but I cannot telnet the port 21 of the private address where the FTP server is (or any TCP port actually) The ftp server is working fine when I connect to its static public elastic IP address but I'd like to not to have to get out of the VPC network. Any idea?

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