Vezon, a colossi driven mad by Karazhni, uses his mask power to fuse the Toa together. He escapes to continue his scheme, leaving the Toa disoriented in their new transformed states…Later after the Toa have learned to work in harmony (unity), they use their new combined power to defeat Vezon and use his spear to undo the unnatural fusions but not before he finished his task.
They could have to encounter Vezon several times before they finally defeat him and each time he could switch up the fusions. In the end though, the Toa would revert back to their natural selves because having the power of two or more Toa in one body at once was destroying them.
This is one example of how we could make the Toa Kaita work in the story.