[JamOfBoy]

To elaborate, everyone has a sub-attack option regardless of whether they have two weapons or not. You can have only a tome equipped, and you have the basic tome attack, and a 'sub attack' option, as though you were using Mixed Martial Arts from Martial Artist. Even if you don't have martial artist equipped. There's no other caveat - you just have the sub attack option, always.

Trying to use this sub attack will simply do a basic attack with your main hand weapon. There are some janky effects, such as multi-round guns only hitting once, when you do this.