Beginning in version 9, the software supports setting up a backup payment gateway. In the event that your primary gateway doesn't respond to transaction requests (ie the gateway is temporarily down, there's a routing issue, etc) the software will 'roll over' to the backup gateway and try the transaction again, to prevent the loss of an order.
Note that this does require that you have a merchant account with both gateways at all times.
2. Set up your primary gateway with the directions found here, but skip the last step directing you to set the active gateway on the Config Wizard page.
3. Set the PaymentGatewayBackup AppConfig to the gateway you want to use as your backup.
4. Finally, configure the PaymentGateway.PrimaryRetries and PaymentGateway.BackupRetries AppConfigs as desired. These settings control how many times the software will try to contact the primary and secondary gateways respectively before displaying an error to the customer. Do not set these too high, or the delay may cause customers to give up.
NOTE: Vault/Wallet-type behavior (such as with the Authorize.net CIM integration) does not work on backup gateways.