Samurai who twoweapon a katana and wakizashi should get some small bonus to damage or alignment record or similar. Though since they start the game able to do this, it would have to be a small bonus.

You can twoweapon pick-axes to dig super fast.

The description for the katana says it can be used as a two-handed sword. Maybe twoweaponing could be adapted for this, i.e. use both hands on the weapon for an extra damage bonus based on strength, at the expense of being able to use a shield.

You can wield artifacts in your offhand, but they must be non-intelligent, non-gifted, and non-crossaligned. (Non-wished-for is an option, but may be too exclusive.)