Add Multiple JSON Objects containing a single JSON Array of Strings each, inside a single JSON File using Java

I'm working on a Java Project that involves a GUI using FXML and I'm currently stuck on the I/O part of the project which involves writing Arrays containing Strings to a single JSON File, I have used the JSON-Simple Maven Dependency from Google.

This is what I have currently done so far for the File Writing part:

    @SuppressWarnings("unchecked")
    @FXML
    public void submitCardio(ActionEvent event) throws IOException, ParseException {
        //set file variable with file path
        File file = new File(cardioFileName);
        //get text field user input from GUI
        String nameField = cardioNameField.getText();
        //get text field user input from GUI
        String durationField = cardioDurationField.getText();
        //declare and initialise Array
        String[] cardioArr = new String[2];
        //define array indexes
        cardioArr[0] = nameField;
        cardioArr[1] = durationField;
        //declare JSON Object
        JSONObject jsonObject = new JSONObject();
        //declare JSON Array
        JSONArray jsonArray = new JSONArray();
        //iterate over indexes of String array and adding strings to JSON array
        for (int i = 0; i < 2; i++) {
             jsonArray.add(cardioArr[i]);
        }
        // Creating JSON Object with cardio key and containing the JSON Array
        jsonObject.put("cardio", jsonArray);
        
        /**
         * Try block which includes the code that appends to File (FileWriter)
         */
        try(BufferedWriter bw = new BufferedWriter(new FileWriter(cardioFileName, true))){
            String jsonString = jsonObject.toJSONString();
            bw.append(jsonString);
            bw.newLine();
            bw.flush();
            bw.close();
        } catch(IOException e) {
            e.printStackTrace();
        }
        
        final String addFile = "/MainMenu.fxml";
        final Parent root = FXMLLoader.load(getClass().getResource(addFile));
        Scene scene = new Scene(root);
        Stage stage = (Stage) ((Node) event.getSource()).getScene().getWindow();
        stage.setScene(scene);
        stage.setTitle("Main Menu");
        stage.setResizable(false);
        stage.show();
    }

I have tried writing to the JSON File however there are Syntax errors once I have added more than a single JSON Object.

Single FileWriter Iteration output below:

This is the Image of the JSON File containing a Single JSON Object (only one FileWriter iteration, CORRECT Syntax)

Two FileWriter Iterations output below:

This is the Image of the contents of the JSON File containing Multiple JSON Objects containing a single Array of Strings Each, (two FileWriter iterations, WRONG Syntax)

I'd like to be able to add Multiple JSON Objects without resulting in wrong syntax in the JSON File, as of now Single iteration of writing to file works correctly, however multiple attempts to write JSON Object to file result in Syntax errors, included below:

JSON Validator Results of Multiple JSON Objects added upon Two Iterations of FileWriter

Any help will be appreciated, I'm still new at JSON and Java; for any further queries comment below.