I agree that Mace defeated Sidious for reals and did not let Mace win. But Sidious showed his cunning in the situation when Anakin entered. He used the force lightning to make himself look pitiable, secure that Anakin would be too emotional to notice that Sidious - not Mace - was the one who was generating the lightning. And he calculated correctly that it would push Anakin over the edge to intervene against Mace, and that Anakin was messed up and naive enough to think he had made an irrevocable choice.
I don't think that exact scenario was his plan all along, but Sidious was a master of setting plans into motion that he could twist to his advantage whichever way things turned out. Whether the separatists or the republic won did not matter. Whether Dooku or Anakin won did not matter. Whether Vader or Luke won did not matter. (Except that he did not foresee Luke taking a third path - laying down his saber.)
But even granting for argument's sake that they stalemated until Sidious threw the fight, then adding Yoda should do the trick, since Sidious would be fighting two guys that he couldn't defeat singly.