One Player must host, the other Player join the hosted Game.
Hosting a Game means - You must do Port Forwarding. You must be able to forward a port in your Router. There are 2 Options. And maybe you must also allow the Port in your Firewall. Important, for Tekken 3 Online it is the Port ''UDP 27886'' !!!
1. Option - Router
Here I cannot help you exactly because it depends on your Internet Provider/ your Router ( Name, Type...). You must login in your router and then you can see
''Port Forwarding''. I have to say it will just take 5min if you have the access/ able to login in your router ^^.
2. Option - Hamachi ( When Option 1 is too hard or impossible to do )
Here you will need ''Hamachi''. It works 100%, but without Hamachi - Kaillera P2P is a little bit better. So if you cannot host you will have to play with Hamachi.
Download and Install it. Create a room and ask an other Player to join. So everyone has an own ''HAMACHI IP ADDRESS''.