Use a vegetable peeler or such to remove the lemon rind and place in one jar.
Cut Key Limes in half, squeeze juice through a strainer to catch the seeds, place juice and squeezed fruit into the other jar.
Fill each jar to within ½ inch of the top with alcohol and lightly hand tighten the lids. You want the air to escape as the jars heat up so do not overtighten.
Place in sous vide bath 130 degrees for two hours.
Optional; Remove from SV, tighten lid and let sit inverted overnight to clarify.
Prepare heavy syrup. Boil 4 cups filtered water, add 8 cups of sugar, stir until dissolved and let cool to room temp.
Pour your now flavored alcohol through a strainer into a mixing bowl.
Do not be shocked. Adding water will cloud the Limoncello, a natural reaction to water meeting the oils. This is a personal taste step. We added 12 ounces of syrup to the Limoncello and 16 ounces to the Key-Lime-n-cello to reach a sweetness we liked. Carefully taste for sweetness, keeping in mind this is very high proof still. Add filtered water to double the jars original volume, then add one more cup. This will take you to about 80 proof, 40 % alcohol which is a standard level for most liquor.
We bottled in pints. Enjoy served over ice, or mix with soda or iced tea. Enjoy!