They're two handed weapons that basically count as 1h x 2 weapons. They were also in NWN1, but not in NWN2. Daelan, the companion, used one:So double-edged weapons count as two attacks even when they're a one-handed weapon? Does that mean someone dual-wielding two double-edged weapons gets 4 attacks?
Were those ever in any edition of D&D?
*Edit* I see they do count as 2 attacks even when one-handed, but the character has to have the two-weapon feat or they suffer a very significant penalty. Obviously that's bugged now anyways.
http://www.gamebanshee.com/neverwinternights/henchmen/daelanredtiger.php
Basically, a 1d8+5 double edged weapon will count as a 1d8+5 weapon in each hand, without usual penalties for medium sized weapons. All other dual wield feats, bonuses and what not apply.
They generally can't be used by dex-based dual wielders though, as Weapon Finesse does not apply, so you need some sort of weird, Strength based character to use them, such as Rangers that gets dual wield feats for free even if they don't meet the requirements.