Reading Taler's website (https://www.taler.net/) it is not clear to me whether customer and merchant must agree to use the same exchange in order to perform a transaction.

It seems to be implied by the key interactions of a Merchant: "Create a reserve based on an incoming wire transfer from a customer" and "Execute wire transfers to merchants in response to validated deposits".

If this is indeed the case, then there's an attractor towards centralization: both customers and merchants will be forced to sign up with the dominant exchange for maximum interoperability.

Either exchanges are given an incentive to federate with zero friction for users, or the Taler network will inevitably become a centralized monopoly.