lambda.tf

# ...
resource "aws_lambda_permission" "with_sns_1" {
  statement_id  = "AllowExecutionFromSNS"
  action        = "lambda:InvokeFunction"
  function_name = "${aws_lambda_function.source_to_target_1_sync.function_name}"
  principal     = "sns.amazonaws.com"
  source_arn    = "${aws_sns_topic.s3_fanout.arn}"
}

resource "aws_lambda_permission" "with_sns_2" {
  statement_id  = "AllowExecutionFromSNS"
  action        = "lambda:InvokeFunction"
  function_name = "${aws_lambda_function.source_to_target_2_sync.function_name}"
  principal     = "sns.amazonaws.com"
  source_arn    = "${aws_sns_topic.s3_fanout.arn}"
}