Speaking in terms of the I-VI Saga, i think it is indicated that Yoda never believed in eventually using the twins to take on the Emperor. He wants them split up simply so the Emperor cannot get them, with no hint ever given that he has plans for eventually training them. Obi Wan however, perhaps feeling a connection to Luke because he saw so much of Anakin in him, personally oversees his care as a hermit living nearby, and thought that in time Luke could become the great Jedi that his father could have been. Owen of course resents Obi Wan and seperates the two as best he can, and perhaps it can be said that Obi Wan might have believed in a radically unorthodox revision of the Jedi code where Luke could be trained not as a child but as an intelligent young adult. In any case, Obi Wan and Luke don't have substantial contact with one another until ANH, whereupon Obi Wan obviously feels that Luke is at a right age to begin training (as he says of the lightsaber, he wanted him to have it "when he was old enough"--implying that he may indeed be breaking from the child-brainwashing methods of the PT jedi). Luke's relatives are coincidentally killed, leaving Obi Wan to take Luke under his wing finally and begin formal training. After he is killed Obi Wan must felt that, perhaps because his spirits existence in the material world is temporary or limited, that Yoda would be the only choice to make a feasible mentor, so he instructs him to seek out Yoda on Dagobah. Yoda of course never agreed to any of this and was against the idea from the outset, but perhaps because at that point there truely wasn't any other option or hope, he reluctantly took on Luke at the persistence of Obi Wan and Luke.

Thats the way i read the new revision anyway.