Ah, I apologize for not providing the rest of the info.
First, get Rebirth. Go here:
http://www.dxx-rebirth.com/
Download the correct package (D1 or D2 for your OS) and extract it directly to your D1/D2 directory. Then set up YANG to point to rebirth in the Settings->Source Ports menu.
When you install rebirth, make a new pilot file to avoid compatibility errors.
Once in the lobby (and you both have the maps, sometimes YANG goes wonky with maps so I suggest backing up all missions beforehand and having the missions folder up in the background so you can replace any maps that get "misplaced"), and both players "ready up," the host clicks "Launch game."
Descent should open for both players. In rebirth:
-Multiplayer tab
-Host -> Start UDP/IP network game
-Set up the game and hit "enter" when the name of the game is highlighted to host.
-Hit enter to start the game.
-Everyone else -> Join UPD/IP network game
-The correct IP address should be pre-entered. Hit enter.
-Wait for the game to appear. Hit enter when it is highlighted and wait for the host.
And that should be it. Have fun, and hopefully I'll see you in the mines (Kali is the best place to find people to play with as of now, but I'll try and set up some YANG matches every once in a while... maybe I should make a thread on the forums here)!
Music1an