This is dumb, the Jedi would get killed. The Emperor would shoot lighting at Obi-Wan/Yoda and hold him off for a bit. Meanwhile, Exar Kun would fight Obi-Wan/Yoda, and would either kill Kenobi in a matter of moments or kill Yoda in more prolonged duel. Now, if he fights kenobi, Yoda might kill the aged and weakened Emperor, but not before Kun kills Obi-Wan and proceeds to kill Yoda.
Return of The Jedi Emperor is stronger in The Force than his version in Revenge of The Sith, but probably a lot less agile and skilled physically. Having said that, Obi-Wan is also an old man. And I think no one will dispute the fact that Palpatine is far more powerful than Obi-Wan. Yoda also seems less physically able than he was in Revenge of The Sith. Yoda and Palpatine would probably fight to a stalemate. Exar would own Obi-Wan. Palpatine would own Obi-Wan. Obi-Wan dies in this, either way.
Then Yoda and Palpatine would still be locked in a battle. Exar would help Palpatine and Yoda would get his ass kicked by both Dark Lords.