Thanks so much for your question.
The UI for quickly adding access keys found here https://circleci.com/gh/$ORG/$REPO/edit#aws
only supports being able to add one set of keys in.
If you need to have multiple credentials, the best way to handle this would be by using Environment Variables.
For instance you can add via the UI (they will be encrypted) here: https://circleci.com/gh/$ORG/$REPO/edit#env-vars
AWS_DEV_ID
AWS_DEV_KEY
AWS_PROD_ID
AWS_PROD_KEY
Then, during the deployment steps you just use those ENVARS in your scripts.