Sprinkle the yeast and 1/2 tsp of the sugar over very warm water in a 1 cup measure. The water should be comfortably warm when dropped on wrist. Stir to dissolve. Let stand until bubbly, about 10 minutes.
Combine 2 cups of the flour, salt, lemon rind and remaining sugar in large bowl and make a well in the center.
Beat the eggs and egg yolks in a small bowl just to mix. Pour eggs. yeast mixture and warm milk into the well. Stir liquids into flour until smooth. Beat well.
Add the softened butter gradually, beating well. Stir in 1 more cup of the flour. Bet until the dough leaves the side of the bowl.
Turn out onto lightly floured surface and knead until smooth and elastic, about 10 minutes.
Place dough in buttered bowl, turning to bring the buttered side up: cover and let rise in a warm place, away from drafts, 1 and 1/2 to 2 hours until it has doubled in volume.
While dough is rising, make the cheese filling. Beat the cream cheese and cottage cheese in a small bowl with an electric mixer until smooth. Beat in egg yolk and sugar. Stir in lemon rind.
Make crumb topping: Combine the nuts, flour, butter, sugar and cinnamon in a small bowl.
Grease two 8 by 1 and 1/2 inch layer pans.
When the dough has doubled, punch down; knead in raisins; divide dough into four equal parts: press two parts into the bottoms and up about 1/2 inch on the sides of each of the layer pans; spread each with about 1 cup of the cheese filling.
Shape the remaining dough into 8 inch circles; place on top of cheese filling. Press spoon handle into dough round the edges to seal.
Sprinkle half the crumb topping over each Babka. Let rise in warm place until dough reaches the top of the pans, about 1 hour.
Bake at 350 degrees for 40 minutes or until the cakes sound hollow when tapped. Turn out onto wire rack to cool. (Place foil loosely over crumb topping; invert onto rack, then turn right side up.) Let cool at least 30 minutes.