Guess no one is interested in these things. Wish those would've increased Fourze's ratings.
Here's what I think if both of those were the same character.....
The zodiarts rider & Ophiuchus horoscope will be the same person. He is the silent student in Ama high. He considers Gentaro's ideals of friendship ridiculous especially with him being Fourze. Believing Kengo is worthy, he doesn't know about the illness. He has feelings for Sonoda, even though he knows she's Scorpion. As the zodiarts rider, he works for the Horoscopes temporarily, just testing Gentaro as Fourze, doing this of his own free will.
As the zodiarts rider, he has the power to sense Zodiarts and fights them. Once a zodiart gets defeated, he clicks the switch and he along with his belt absorbs its energy and gaining their abilities.
The Ophiuchus zodiarts power gets addicted to the zodiarts rider as it slowly eats away his humanity. He tries to control the power believing the powers would give his human self perfection. We can think of that as both Blade King & Joker story.