I played through HOTU in MP. It can be done with a bit of fuss. The cutscenes dont work properly, otherwise its fine. IIRC we had to play the end of chapters 2 and 3 in SP and then do some exporting/importing of characters in the beginning of chapter 3.

XP gains will also be a bit smaller. I think I was at level 13 after SoU in MP (compared to lvl 16 when I played solo)and didnt reach that high a level in HOTU. It's not that much of a deal towards the end though, as an extra INTELLIGENT companion more than makes up for losing a 3-5 levels, and the benefit of each extra 20+ level is relatively smaller than it would be earlier in the game...

Still I used LETO to make us start HOTU at level 16. I'm not that fond of classical dungeon crawling so I found chapter 1 the least enjoyable part, and it can be quite challenging if you are too low a level...

Hmmm, Hordes starts you out at lvl 15.

15 then, my memory is rather sketchy as it is almost a year since I played it:) We played through SoU first and ended up around level 13, and then I edited the characters to have the same experience as the pre-made Hordes characters that come with the expansion.

I found some oddities in XP distribution in NWN MP. One player would (inconsistently but the gap tended to grow over time) get more XP than the other, and it wasnt that the lead person always got an extra XP for kills giving an odd number of points.

I also recall the first time we tried NWN MP (abandoned since one of our PCs couldnt handle it) and seeing the higher level character get MORE xp due to some sort of stupid henchman penalty rule punishing low-level characters with higher-level allies). Didnt notice that in our later ventures as we both were semi-tanks of equal level.