An api key, with a usage plan (limit "2"), is accessing my api, but the default limit of this endpoint/stage is "1".
Has the user with the api key then a limit of "1" or "2"?
https://docs.aws.amazon.com/apigateway/latest/developerguide/api-gateway-request-throttling.html
The default stage throttling setting is a limit of api requests across all users of your apis. If that is set to 1, than in total only 1 request per second will be served to all your users. A usage plan does not override this value.