Suppose two pupils heard hadith from Shaykh Yūsuf. The earlier pupil, ʿUmar, died in 450 AH, while the later pupil, Ḥasan, lived much longer and died in 520 AH. Their common teacher makes the former sābiq and the latter lāḥiq.
Shared teacher: Yūsuf.
Earlier pupil: ʿUmar — died 450 AH.
Later pupil: Ḥasan — died 520 AH.
Gap: 70 years.
These dates and people are hypothetical teaching data. In historical application, one must verify birth and death dates, the period of hearing, and each pupil's actual contact with their common teacher.