Is fallback useful with only one acquirer?

Partly. With a single acquirer, retries can handle transient technical errors, but rerouting needs at least two providers able to process the same payment. Most operators add a second provider for exactly that reason, often one with local acquiring in their largest market.

Retries, yes; rerouting, no

With a single acquirer, fallback can still retry a payment after a transient technical error, such as a timeout or a gateway error, on the same provider. That recovers some payments. It cannot reroute to another provider when the acquirer is down, declines a card type it handles poorly, or has an issue with a specific issuer.

What a second provider adds

• Recovery during an acquirer outage.
• A second chance on soft declines where issuer behavior differs by acquirer.
• Local acquiring in the largest market, which improves approvals on domestic cards.
• Negotiating leverage with the first acquirer.

The practical step

Most operators add a second provider for fallback, often one with local acquiring in their main market, and route a share of normal traffic to it so the connection stays live and tested. Stored cards must be usable on both, which network tokens and a universal vault provide. Reporting then shows recovered payments per month, which is usually enough to justify the second contract. Fallback with one acquirer is a safety net with a hole in it; the second provider closes the hole.