Wash the lemons with a vegetable brush and hot water to remove any residue of pesticides or wax; pat the lemons dry.
Carefully zest the lemons with a zester or vegetable peeler. Only take the yellow rind part, not the white pith underneath. The pith is very bitter and will spoil the flavor of your limoncello.
In a large glass jar (1-gallon jar), pour in one bottle of vodka. Add the lemon zest as it is zested.
Cover the jar and let sit at room temperature for at least 10 days and up to 40 days in a cool dark place. The longer it rests, the better the taste will be. (There is no need to stir – all you have to do is wait.)
In a large saucepan, combine the sugar and water; cook until thickened, approximately 5 to 7 minutes. Let the syrup cool before adding it to the limoncello mixture.
Add to the limoncello mixture from Step One. Add the additional bottle of vodka. Allow to rest for another 10 to 40 days.
After the rest period, strain and bottle: discarding the lemon zest. Keep your bottles of limoncello in the freezer until ready to serve.