Does rotating two pairs actually make them last longer, or is that a story that sells two pairs?

The longevity part is real and it is smaller than the way it gets repeated. The better argument for rotating is a different one.

Why foam recovers. Midsole foam is compressed on every stride and it re-expands afterwards, but not instantly - full recovery takes considerably longer than the few hours between an evening run and the next morning. A shoe run daily is being compressed while still partly compressed from yesterday, and that accelerates the permanent loss of cushioning.

So giving a pair a day or two between runs does extend its useful life. How much is genuinely not well established, and anyone quoting a precise percentage is making it up.

The stronger argument is variety. Two different shoes load your feet and legs slightly differently, different stack, different drop, different geometry. Running everything in one shoe means the same tissues take the same load every time, and varying it is generally thought to be a reasonable idea for anybody running regularly.

And the boring practical one: wet shoes. At forty kilometres a week you will get caught out, and a soaked pair takes a day or more to dry properly. A second pair means you are not running in wet shoes or drying them badly on a radiator, which does wreck them.

If you do buy a second pair, buy a different shoe, not the same one again. That is where most of the benefit is and people routinely buy a spare of what they have.

A sensible pairing for someone at your volume:

A daily trainer, cushioned, durable, unremarkable. Most of your running.

Something lighter and firmer for faster sessions, if you do any. Or a second daily trainer with a noticeably different feel if you do not.

A trail shoe instead, if some of your running is off road. That is a different purchase solving a different problem and it also gives you rotation for free.

What to avoid: two shoes with very similar geometry but different enough drop that you are switching between them without noticing. A large drop difference is worth introducing gradually rather than alternating from day one.

On how to rotate: alternate rather than running one until it is worn. Hard sessions in one, easy in the other, is the usual arrangement and it happens to give each pair recovery days automatically.

And keep the older pair in service after buying the new one rather than retiring it: that transition period is when you find out whether the new shoe suits you, while still having something known to run in.

Forty a week in one pair is where this starts to matter. Under twenty and none of it is worth thinking about.

Wet shoes is the whole argument honestly.

On replacement, since you said you replace when they feel dead - that is a better method than the mileage numbers everyone quotes, and worth defending.

The commonly repeated range is very wide in practice. It depends on your weight, your gait, the surface, and enormously on the shoe. Some lightweight racing foams are done in a fraction of the mileage of a firm daily trainer. A single number applied to all shoes is not useful.

Better signals:

How it feels on a longer run, not on the first kilometre. Dead cushioning shows up as unusual leg soreness after distance before it shows up as anything you can see.

Compression lines in the midsole, and whether the foam still springs back when you press it hard.

Wear pattern through to the midsole at the outsole.

Upper failures, often what actually ends a shoe rather than the foam.

If you do want a number, log the mileage per pair so you learn your own figure for the shoes you actually buy. After two or three pairs of the same model you will know it better than any general advice, and rotation makes that logging slightly more work and considerably more useful.