Well, the quarterstaff is a double weapon as well, according to D&D rules. So, in essence, the double sword is just a 1d8 quarterstaff.
Maybe you have to check the options on the weapon, and there is a way to toggle between one-handed fighting and two-handed fighting. Or maybe in the inventory screen, you have to drag the two-handed weapon to your off-hand slot. That sounds kinda reasonable now that I think about it.
And please don't tell me the quarterstaff isn't a double weapon! The manual says,
A large, stout piece of heavy wood. Sounds like a club!!
Hopefully someone will fix that if the quarterstaff is not a double weapon.
*edit*
I was just looking in the weapon tables in the back of the NwN guidebook, and for quarterstaff damage, it has "1d6/1d6." This seems to indicate it is a double weapon in NwN, since all other double weapons have the same "1d?/1d?"....