How can I establish a TCP connection between many computers behind two firewalls?

Answer 1

You could have some server in the cloud acting as a proxy for your connections. For instance, think about how a service like LogMeIn or GoToMyPC manages connections between a client and the controlled host. This is one idea.

Answer 2

This should be an interesting read.

http://stackoverflow.com/questions/1539339/how-skype-work-without-port-forwarding