Artifact pair of scimitars (probably with slightly different names) that grant bonuses of some sort when used in two-weapon combat.

Extend the weapon wielding system to have a third slot for “alternate weapon, not wielded” when already twoweaponing, and provide a way to switch between the two sets. This would allow you to keep a launcher ready and switch to and from it with minimal friction.

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.)