First, upload test files to an S3 bucket you control (replace YOUR_BUCKET_NAME) in two directories representing isolated namespaces (123 and 124).
echo "123" > test.txt
aws s3 cp test.txt s3://YOUR_BUCKET_NAME/123/test.txt
echo "124" > test.txt
aws s3 cp test.txt s3://YOUR_BUCKET_NAME/124/test.txt