One extra intermediary in an isnād admits several possibilities: the addition may be mistaken; the shorter chain may conceal a gap; or both routes may be authentic if the narrator received that report both directly and through another person.
A practical procedure:
1. Establish the transmitters and the chronology of both chains.
2. Examine transmission formulas, especially “I heard” and “he told me.”
3. Evaluate evidence of hearing for this particular report.
4. Compare the narrators' precision, especially among a teacher's pupils.
5. Reach a conclusion based on the actual evidence.
Neither “the chain is longer” nor “the chain is shorter” proves a defect. Evidence of hearing and comparative weighing are indispensable.