In one of the best scenes in Episode II, Count Dooku makes an appeal to an imprisoned Obi-wan to join him and destroy Darth Sidious together. In actuality Obi-wan refused, but let’s imagine he did decide to join Dooku.
Obi-wan is missing, presumed dead. For the next 6 months as the clone wars begin Count Dooku trains Obi-wan in secret for their planned confrontation with Sidious. Dooku concentrates on improving Kenobi’s bladework but his appeals for his new apprentice to draw on the dark side go unheeded. Dooku resigns himself to improving Obi-wan’s power with the force within the constraints of the light side, while continuing to utilise the dark side himself (and pointing out, to no avail, how much more powerful he is by using it). The training is specifically geared towards confronting Sidious, so Dooku spends much time showing the best methods for dealing with force lightning and impresses upon Obi-wan that he must be prepared for Sidious’ incredible speed.
The actual confrontation occurs on Coruscant. In a scheduled meeting with Sidious in the works, Dooku and Obi-wan openly approach Sidious together. There is no surprise attack as such, beyond Sidious’ initial surprise that his apprentice is just about to try and kill him. In this regard the initial setup resembles the confrontation between Dooku and Obi-wan/Anakin on the Invisible Hand. Given that the confrontation occurs in the works, there is nobody else around.
I'm not sure Obi and Dooku singularly are a match in the Force, so they would have to try and close the gap very quickly. Sidious wiped the floor with Maul and Savage rather easily, so the benefit of having Kenobi is questionable. After that, it's just Dooku and he loses.
A Dark Side Kenobi is too speculative and hard to quantify. I personally think it wouldn't benefit Kenobi turning to the darkside as his whole style is about embracing the light side and keeping a cool head during even the most intense situations.
And looking at how Kenobi got thrashed by Dooku even with help from the Chosen One, I don't see him making much difference here, unless people think Dooku has the power to save Kenobi from Sidious's Force attacks.
We know Sids can penetrate his defense. This is assuming Sids doesn't just chuck him like a used toy. Then it's all on Dooku, and his chances aren't that good.
Their only chance is to overwhelm him in Sabers. Leave any distance or stop in the Saber fight, then Kenobi goes instantaneously flying.
As for Sabers, Kenobi's Soresu would be useless. His Ataro/Jar Kai combo he used against Maul/Opress would be his best bet here. But if I were Dooku I'd take Anakin over Kenobi for this fight.
I'm pretty confident that, given how easily Dooku can TK Kenobi, Sidious will not have much trouble. Unless the premise of this thread implies that Dooku teaches Kenobi to train specifically to kill Sidious.