From my experience with plying Infinity engine games on multiplayer IWD2 was so far the most painful one (I've also played original BG1, BG2 and IWD1). It seems that IWD2 has quite few glitches and bugs regarding multiplayer that have never been fixed.
For example there's a fight with orc and goblin shamas before entering the stronghold. One of the shamas will try to cast a call lightning spell, which has quite huge delay after casting. You can only notice that when you follow the combat log. Unfortunately we saved the game after the shaman was dead, but before the lightning struck. Later on we had to reload the game, but due to that spell the save file was corrupted and we couldn't use it and had to use different save prior to the fight (we then made sure that he won't be able to finish casting any spell).
Yet the worst thing is that the further we got into the game the loading times and people being dropped, or unable to join were getting worse and worse. We thought that there might be something with the connection, but when we used a save from the beginning everything was working.
For those reasons I can't recommend playing IWD2 on mp and I've read on other forums that people were also unable to finish the game. I would recommend trying IWD1 instead, cause this one is working great on mp, you can check wether that one is working properly and it's one of the best multiplayer experences that I had. We were able to finish the main game, HOW and TotLM (with some minor mods) without any major technical issues.
