Two separate canals in the distal root of a mandibular molar in my practice is more common than what is reported in the literature. Learning the micro anatomy of aberrant root forms, using the surgical operating microscope for the whole procedure, scheduling enough time, and an attitude to excel help greatly in finding these canals. Over 20% of all the mandibular molars that I treated in the past couple of months have two separate canals in the distal root. If I add the cases that had two canals that joined apically, then the numbers are close to 40-50%.
Here are a few examples of two separate canals in the distal root of mandibular molars: