In a large saucepan combine the rutabaga, the potatoes, the sugar, and salt and pepper to taste with enough water to cover the vegetables by 2 inches. Bring the water to a boil and simmer the mixture, covered, for 30 minutes, or until tender. Drain the vegetables. In a food processor or with food mill or ricer puree the vegetables, in batches, and transfer the puree to a bowl. Add the butter, salt, and pepper, to taste and let the puree cool to room temperature. Transfer the puree to a freezer container with a tight fitting lid or to a reseal-able freezer bag and freeze the puree for up to 1 month.
To reheat: Defrost the puree in the refrigerator. Pour off the liquid that has separated from the puree and discard it. Transfer the puree to a buttered saucepan and simmer it over moderately low heat, stirring occasionally, for 10 minutes, or until it is heated through.