HotU is actually designed for single-player only. It will not work properly in multiplayer. You can make it work in multiplayer, but it requires a bit of hacking.
We're progressing through chapter 2 and the only problems we've had so far are cut scenes teleporting people to strange places, and plot items being divided between multiple players. If you do play multiplayer, make sure one of you knows how to use the consol commands, just in case.
