Combine pectin and 1/4 cup sugar in a 5- to 6-qt. pot. Stir in cherries and rhubarb, then cherry and lemon juices and butter. Bring mixture to a brisk boil over high heat, stirring often.
Add remaining 1 1/4 cups 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, inverting jars occasionally to distribute fruit. Keeps, chilled, up to 1 month.
*We used Trader Joe's Just Cherry juice.
Note: Nutritional analysis is per tbsp.