Partners

Dual wielding Elsinore Swords was not purposefully implemented, it's a glitch. And are you sure all partners add a commerce slot? Or just Menma?

I was unaware of the glitch of the Elsinore swords, I was merely going off the picture that my friend showed me from the JP servers.

And yes, both Partners get 1 extra slot and an additional 100 weight limit (so 1000 for Horses, 1800 for Elephants).