Combine pectin and 1/4 cup sugar in a 5- to 6-qt. pot. Stir in berries, then berry and lemon juices and butter. Bring mixture to a brisk boil over high heat, stirring often.
Add remaining 1 cup sugar. Return jam to a brisk boil, stirring. Cook, stirring constantly, 1 minute.
Remove from heat.
Ladle jam into heatproof jars and close with lids.
Let cool to room temperature. Keeps, chilled, up to 1 month.