Agreeing with the drying-time point and adding the trick that let me drop the second pair entirely.
Wet shoes are mostly a problem because the inside stays wet. Stuffing them tightly with newspaper or any dry absorbent paper, changed once after an hour, dries them enormously faster than air alone — the paper is doing the wicking that the air cannot do inside a shoe. It is the difference between wearable by morning and wearable the day after.
If you carry a second pair, the two failure modes I would warn about:
The useless-but-light option. Thin flip-flops or packable slippers weigh nothing and cannot actually replace your main pair for a day of walking, so when it matters you are still in wet shoes. They are a shower and hostel-floor item, which is a real but different job.
The identical-purpose option. Two pairs of the same kind of shoe is a third of your bag spent on redundancy. If you take two, they should cover different situations — one that can get wet and dry fast, one that cannot.
Mine ended up as a light trail shoe worn on the plane and a pair of thin sandals that pack flat. The sandals are not a walking shoe and I do not pretend they are.