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.