It's just great cooperation...
It's easy.
Kenko and Tokina has the same owner. Kenko and Tokina cooperate with Hoya. And it's all.
This companies are not in structure of HOYA.
Kenko sells (and perhaps makes) film cameras, digital cameras and camcorders under the Kenko brand, and makes lenses under the Tokina brand. It sells all sorts of camera accessories under the Kenko brand, including tripods, filters, conversion lenses, etc. It also makes accessories sold under other brands, such as the Slik and Daiwa tripods, the Cokin and Lee filters, the Tamrac camera bags, ATP memory cards and Aska portable image viewers.
From
Kenko - Camerapedia.org