Prepare for water bath canning: Sterilize the jars and keep them hot in the canning pot, put a small plate in the freezer, and put the flat lids in a heat proof bowl.
Put the rhubarb, strawberries, sugar and 1/2 cup water in a wide, 6 or 8 quart preserving pan. Cook over medium-high heat, strirring frequently, until the juices just cover the fruit (10 to 15 minutes).
Pour into a colander set over a large bowl and stir the fruit gently to drain off the juice. Return the juice to the pan and bring to a boil over high heat. Boil, stirring occasionally, until the the syrup is reduced to about 1 1/2 cups, about 20 minutes.
Return the fruit and any accumulated juice to the pan, along with the lemon juice. Bring to a simmer. Simmer, stirring frequently, until a small dab of the jam spooned onto the chilled plate and returned to the freezer for a minute becomes somewhat firm (it will not gel), about 15 minutes.
Skim off as much foam as you can, then remove from the heat and stir gently for a few seconds to distribute the fruit in the liquid.
Ladle boiling water from the canning pot into the bowl with the lids. Using a jar lifter, remove the sterilized jars from the canning pot, carefully pouring the water from each one back into the pot. Place them upright on a folded towel. Drain the water off the jar lids.
Ladle the hot jam into the jars, leaving 1/4 inch headspace. Use a damp paper towel (or cloth) to wipe the rims of the jars, then put a flat lid and ring on each jar adjusting the ring so that it's just finger-tight.
Return the jars to the water in the canning pot, making sure the water covers the jars by at least 1 inch. Bring to a boil, and boil for 5 minutes to process.
Remove the jars to a folded towel (on a cookie sheet) and do not disturb for 12 hours. After 1 hour, check that the lids have sealed. If it's not sealed, eat!