Preheat oven to 200°C (392°F). Grease two loaf tins and line with parchment paper.
In a large bowl, mix together the flours, sugar, baking powder, baking soda, and salt. Rub in the margarine until it resembles breadcrumbs.
In a separate bowl, whisk together the eggs and buttermilk. Make a well in the flour mixture, and pour in the liquid.
Mix together just until all the flour is moistened, being careful not to overmix.
Divide the batter between the two tins (about 820 g each) and lightly flatten the tops.
Bake in the preheated oven for 30-35 minutes, until a toothpick comes out clean and the top is golden. You want to make sure the bread is fully baked, without becoming dry.
Allow the loaves to cool in their tins for a few minutes before removing to a rack. Once they are only slightly warm to the touch, wrap them well in cling film to prevent drying out.