A double hammer is a double weapon. You can fight with it as if fighting with two weapons, but if you do, you incur all the normal attack penalties associated with fighting with two weapons as if you were attacking with a one-handed weapon and a light weapon.
Cost: 70 gp
Damage: 1d6/1d6(S) 1d8/1d8(M) 2d6/2d6(L)
Weight: 18 lb.
Source: Complete Warrior