Wakizashi is a short sword.
Still a sword, not a dagger.
I said sword and parrying dagger/dagger shouldnt be considered dual wielding, because most of the attacks will be done by sword, rather than dagger.
But with long one handed sword and a short sword, nothing limits you from using both equally, which is practically an essense of dual wielding. Dagger has, in conjunction with sword, more defensive purpose, like shield, only lighter.
It is as if people called using sword and very small buckler, or duelling glove dual wielding. You can still attack with buckler but you sure as hell not going to use it in the same manner as sword, and that is same for dagger.