We've seen Sith and Dark Jedi such as Asajj Ventress, Count Dooku, etc. serve alongside the Separatist forces in the Clone Wars. Is there evidence to suggest other Dark Jedi or Sith, specifically named or otherwise, joining the Separatists?
In the canon novel Dark Disciple, written by Christie Golden based on unfinished arcs of Star Wars: The Clone Wars, Jedi Master Quinlan Vos temporarily joins the Sith during the Clone Wars. He was sent by the Jedi Council in the final months of the war to train alongside former Sith assassin Asajj Ventress in order to assassinate Dooku. However, in an effort to learn the identity of Dooku's master, Darth Sidious, Vos succumbed to the dark side and joined up with Dooku, killing clones and Jedi along the way. Quinlan's love for Ventress convinced him to return to the light, but he meets your terms.
Additionally, I would count Prosset Dibs as a Jedi that fought against the republic. In the canon comic miniseries Jedi of the Republic--Mace Windu, set in the early days of the Clone Wars, the blind Jedi Master grows disillusioned with the Jedi's involvement in the war and expresses his feelings through violence. He is defeated by Mace Windu and sent to work in the archives as punishment. At the end of the war, he becomes the Tenth Brother, a member of the Inquisitorious. While not technically fighting for the Separatists, I would still put him up for consideration.
Legends: there were a lot
There were many more dark siders on the side of the Separatists in Legends than have appeared so far in Canon. Wookieepedia identifies them as "Dark Acolytes", although it's unclear to me whether this is an in-universe name—The Complete Star Wars Encyclopedia, for example, only refers to lowercase "dark acolytes" (eg in the entry for Sora Bulq).
There are 17-18 characters who fit the description, depending on whether Dooku's doppelganger can be considered a distinct character. Some of the more notable Dark Acolytes (besides Asajj Ventress and Savage Opress) include:
Republic: Siege of Saleucami (via Wookieepedia)
Much of the Republic comic series follows Quinlan Vos's actions during the Clone Wars. He's a Jedi Master who is sent to infiltrate Dooku's inner circle, only to find himself actually drawn to the Dark Side. He turns from a double agent to a triple agent, only to resume his status as a quadruple agent, and so on, eventually successfully returning to the light before the end of the war. He later escapes Order 66 on Kashyyyk and has a son.
Sora Bulq is a Weequay Jedi who first appears as a background character in Attack of the Clones, and later reveals in the comic Jedi: Mace Windu that he's joined Count Dooku. Throughout the Republic comics, he goes on to kill Jedi Council member Oppo Rancisis, and is eventually killed by Quinlan Vos.
Sev'rance Tann is a Dark Side-using Chiss and the player character in the CIS (Separatist) campaign in Galactic Battlegrounds: Clone Campaigns. Since it was released concurrently with Attack of the Clones, this game expansion is notable for being one of the first portrayals of the broader Clone Wars, making Tann one of the first Dark Jedi characters appearing in Clone Wars media. She eventually dies in a lightsaber duel with the hero of the Republic campaign, Echuu Shen-Jon.