True about the demon blade mode, although if I recall correctly it was not using the mode itself that caused Black Star to hallucinate, but the fact he pushed himself too far, allowing him to become open to Asura's insanity.

Come to think of it, both Ox and Harvar have similarities to Maka; studious and serious respectively.

I think their respective soul wavelengths would count against Black Star and Harvar. The very fact they're immune to the insanity wavelength could mean they simply can't work with one another; not responsive enough to each other. However, it's still a neat idea. Asura wouldn't know what hit him. :)