In a medium bowl, combine sugars, cinnamon, nutmeg, cloves and salt; whisk to combine.
Place about 1/4 of the apples and pears in the crock pot, sprinkle 1/4 of the sugar mixture over the fruit in slow cooker; toss to combine. Begin layering the rest of the fruit and sprinkle with remaining sugar mixture until cooker is full. NOTE: Fill the crock pot up plus some extra. The fruit will cook down.
Cook on low setting for 10 hours; stirring occassionally. Mixture will thicken and become dark as it cooks.
After 10 hours; stir in the vanilla. Use and immersion blender to puree the fruit until smooth. Continue cooking on low another 2 hours uncovered.
Ladle hot apple-pear butter into hot sterilized jars, leaving 1/4-inch headspace. Wipe rims of the jars clean with clean damp cloth, cover with hot sterilized lids, and screw bands on.
Allow jars to rest undisturbed on countertop for six hours or overnight.Check lids to ensure they sealed.
Apple butter will keep for up to one year in a cool, dark place.