They are beyond Sparda in power, Chen literally got power from the Beastheads. Text literally says this:
"Probably." Dante shrugged. The power that now imbued himfelt neither good nor evil. It simply was. Whoever wielded it could reshape reality to its own design,for better or for worse.
Did you think I would let my research go to waste? And now you've proven to me how to harness the power of the Beastheads. It doesn't hurt those with demon blood."
Dante could sense a change in the other man's aura. He was no longer purely human, but not a demon either. Chen emitted a powerful magic that lacked the wicked blackness of the Underworld. His aura was green with immeasurable lust for power and knowledge.
Strange energy danced around the room, colliding in on itself in a magical storm-Dante and Chen stood calmly in the eye.
Yeah, they both are related to Sparda but the power they wield doesn't come from Sparda, one is literally being powered by the beastheads and the other has already surpassed Sparda's power long ago, the power is his own and a little bit of the beastheads too.