Combine apple juice and 4 tablespoons lemon juice in heavy sauce pan. Peel, core and cut pears, one at a time, into 1/2 inch pieces.
Mix pears into juice mixture in pot as soon as cut, to prevent browning. Cook over medium heat until pears release enough juice for mixture to boil, stirring frequently, about 16 minutes.
Reduce heat to medium-low, cover and simmer until pears are very tender. Stir frequently for 20 minutes. Remove pot from heat. Use handheld immersion blender to puree mixture. Add 2 tablespoons lemon juice, brown sugar, nutmeg and kosher salt. Bring to boil over medium heat, stirring until sugar dissolves. Reduce heat to medium-low and simmer uncovered until pear butter thickens and is reduced to 8 cups, stirring every 5 minutes (about 1 hour).
Ladle pear butter into clean canning jars, leaving 1/4 inch space at top of jars. Remove air bubbles. Wipe jar threads and rims with clean, damp cloth. Cover with hot lids and apply screw bands.
Process jars in pot of boiling water 10 minutes. Cool completely and store in cool, dark place for up to 1 year.