Not sure if this is a known issue, but the Arachnos Mace made to be shown when sheathed on a costume will make one of the two dual pistols made visible on the same costume disappear. So it's either Mace + one pistol, or two pistols and no mace. This will actually make the model for the one pistol completely disappear, too, when the mace is visibly sheathed, and you'll use an imaginary pistol to shoot with in your right hand. Finger guns confirmed! Also, it's gotta be a bug with the mace specifically, because katana can be shown with two pistols just fine, and it doesn't seem to be a matter of a "visible weapons cap."