Beat the butter, oil, and sugar in the mixing bowl of a standing mixer fitted with the paddle attachment until fluffy. Beat in the egg yolks and vanilla extract.
In a separate bowl, combine flour, baking soda, baking powder, salt and lemon zest. In a small bowl, mix together the buttermilk and yogurt.
Mix the dry ingredients into the butter and egg mixture alternately with the buttermilk, stirring just until combined. I use the lowest speed of the mixer. The batter will be thick.
Add the blueberries and apricots and fold them into the batter using a rubber spatula.
Whip the egg whites to a moderate peak using a hand mixer (the peaks should droop a little when you scoop the whipped egg whites with the beater and turn the beaters upside down). Fold the whipped egg whites into the batter in thirds using a rubber spatula. Little pieces of egg white should be visible in the batter. You don't want to fold them in too much.
Pour the batter into the buttered baking pan. Sprinkle with the Streusel Topping.
Bake for 50 minutes or until a toothpick inserted in the center comes out clean. Cool on a rack for 15 minutes before serving. This coffee cake is great warm or room temperature.