Why do many clock faces show two 12-hour cycles instead of one 24-hour cycle?

Answer: Day and night were counted separately

Finger joints made twelve feel natural — Finger counting is a plausible reason the number 12 felt natural, because four fingers have twelve joints. But that alone does not explain why a whole day became two 12-hour halves. The stronger trail is Egyptian daylight and night being counted separately.

Shorter dials were easier to read — Shorter dials are easier to read, and that convenience may have helped the 12-hour face survive. But it does not explain why the day was split into two 12-hour halves. Egyptian timekeeping used separate 12-hour cycles for daylight and darkness long before familiar mechanical dials became common.

Day and night were counted separately ✓ — Egyptian timekeeping treated daylight and darkness as separate cycles, each with 12 hours. The 24-hour day came from joining those two halves. The 12-hour face therefore preserves an older way of counting rather than a modern design choice.