Whisk together flours, wheat germ, sugar, salt, and baking powder in a medium bowl. Blend in butter and shortening with a pastry cutter or your fingers until mixture resembles coarse crumbs.
Stir in 1/4 cup buttermilk with a fork until dough begins to form. Add additional buttermilk until dough holds together when pinched between fingers. Shape dough into a disk; wrap in plastic. Refrigerate at least 1 hour or overnight.