Obiwan: I will take the child and, watch over him. Master Yoda, do you think Anakin's twins will be able to defeat Darth Siddious?
Yoda: Strong the force runs in the Skywalker line. Hope we can...Done, it is. Until the time is right dissappear we will.
And, in the Quigon thing:
Yoda: Failed to stop the sith lord, I have. Still much to learn there is...
Quigon: Patience. You will have time. I did not.
So, basically one can deduce from the screenplay. That they didn't go after Siddious becuase, they were in training with Quigon. Although, I would say that Yoda wasn't as, healthy as he was before the battle. He did get blasted with lighting, had to avoid those floaty circles, got blasted with lighting again, and fell down a distance after all. Obiwan wasn't powerful enough to fight Siddious on his own. So, they had to train with Quigon and, then they couldn't get siddious when they had recovered. Since the Empire was more powerful then before thus, they had to wait. Not only that the thought of the twins were fresh on their minds. So, they would probably give away that to the Emperor too. Then, again it could just be that both Yoda and, Obiwan got sick after the fights and, needed time off
.