Dyon's original plan was to have the demon generals share their soul power like he and Ri did. This required a base level of trust, something they should have had after fighting so many wars together, but, aside from that, it also required expert level control – a level of control that increased manifold for each additional demon general that helped. In addition to this, to lower the level of difficulty of melding souls, the soul strength of the demon generals had to be similar. Knowing this, only lower level demon generals were of any help.
At first, Dyon and Kaeda had only wanted the help of one other demon general. That would have kept the difficulty at the lowest level possible. But, it became clear very quickly that this wouldn't be possible. Even two demon generals couldn't replicate Dyon's efficacy with the ancient Elvin techniques, something that they found absolutely ridiculous, but was true nonetheless.
