Mix rhubarb and sugar in a large heavy pot and let stand for 2 hours; stir in lemon juice and strawberries. Bring the mixture to a boil, stirring to dissolve sugar. Set heat to medium-high and cook jam at a full rolling boil until thickened, stirring often to prevent scorching, 20 to 25 minutes. A candy thermometer inserted into the middle of the jam, not touching the pot, should read 220°F (105°C).

Remove jam from heat and stir for 2 to 3 more minutes; skim off any foam.

Sterilize the jars and lids in boiling water for at least 5 minutes. Pack the jam into the sterilized jars, filling the jars to within 1/4 inch of the top. Run a knife or thin spatula around inside the jars to remove air bubbles. Wipe rims of jars with a moist paper towel and seal with lids and rings.

Place jars on a rack in a large stockpot; fill halfway with water, ensuring water level is 1 inch above jar tops. Cover and boil for 10 minutes to process.