How do I further diagnose TCP connectivity (timeout) problems? (ICMP, DNS work fine)
First step: check that Mac n°2 is connected with the network interface you think it should (i.e. Ethernet and not AirPort or anything else).

If you are using the famous Automatic network configuration you are never sure on which interface your computer is talking or replying.
For example, your Mac n°2 might well be plugged toward the Internet through your neighbour Wi-Fi connection.