Whip margarine and butter in a stand mixer until fluffy. Add strawberries. Mix until well-blended. This may take a LONG time (up to an hour) for all the strawberries to blend into a smooth buttery texture so turn it on and let it mix, use a spatula to wipe down the sides from time to time. Once well-blended, add powdered sugar and mix until well-blended again. Cover and store in refrigerator. Use within 2 weeks.