After spending hours trying to configure it, I don't think it is possible to port forward 6112 to two computers in the same house, meaning we cannot play on battle.net together.

Does anybody know of a workaround? I am currently downloading combat edition to test that.

Yes, you'll have to give each player a unique port and then forward each port to the matching computer.

Here's a tutorial I just found on google from war2.ru http://en.war2.ru/support/how-to-check-your-game-data-port/

You both will be able to play at the same time once one of you changed his port, but you probably won't be able to play in the same game room (It's gamble, some routers support "Hairpinning", some not)

If you both want to play together in the same game room then one of you could use a VPN such as hide.me as a workaround. Or you guys just play LAN games instead :)

Yes, you can install combat, make sure you selected the "random port" option on at least one of computers.
You can't forward the same 6112 port on both computers, but you can configure different ports on different computers. Starting combat using war2ploader (default option) will use uPnP for either default 6112 port or any other game data port configured by the combat installer. So nothing required after installation: just install one combat with default port, another one with random port, then start both and enjoy playing both together.