There is not transaction scope in EventBridge. Each target handles the events it receives in an independent way. As you do need some coordination between them I would recommend that you invoke a Step Functions state machine from EventBridge. The state machine will implement a Saga Pattern to perform both operations and roll back in case of failures.
EventBridge by itself is not transactional.
The Lambda can do both create User in DB and create Coupon. If this fails, you can handle this in your Lambda function and then throw an error.
EventBridge has an retry for failed Lambda functions.
As @Uri explained, this would be more a case for coordinating using Step Functions, here's one example of implementation https://github.com/aws-samples/aws-step-functions-saga-pattern-with-sam
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