Not able to use a Weapon on the right Side of Weapn Set II

So, can anybody help here, why is this red? "You cannot use this Item.Its Stats will be ignored".

It already is a pain in the ass to get this weapon equipped in the first place, only when I put in the Shield on the Set II and Hammer in the Offhand, I can switch, its not possible to put in the Hammer right away, all the strength requirements are fullfilled, so is this a bug or what is the issue here?

When you a swap weapons, does it only swap to one of those weapons equipped or both?