Why didn't Yoda train Leia as well as Luke? He said that the "force was strong with her" as well.
Luke had discovered that his father was a Jedi and that he had some skill with the force from his time with Obi-Wan. He was advised by Ghost Obi-Wan to seek out Yoda to further his training. So he seeks out Yoda for training.
Leia on the other hand does not know of her parentage, had not experimented with any Force training and had no time with Obi-Wan at all. She does not even really understand the Force, never mind seeking out a master for her training.
I think it is also clear that Leia is not as inherently strong with the force as Luke. Vader has her in captivity, even tortures her and doesn't seem to sense her latent abilities. Luke, despite very brief training on the flight to Alderaan, is quickly identified by Vader as being strong, even when he is just another pilot in the battle over the Death Star.
I think Yoda and Obi-Wan view Leia as a backup, to be used if Luke failed, and that to keep her parentage secret from her, Vader and the Emperor was the best way to keep her safe.