public class LogController{
@FXML private TextArea logTextAreaFx;
public void handlelogTextAreaFX(ActionEvent event, String text){
    logTextAreaFx.appendText(text);
}

public class MainController{
@FXML LogController logController;
public void handleChatConsoleButton(ActionEvent event){
    logController.handlelogTextAreaFX(event, "test");
}

<VBox prefWidth="500" prefHeight="300" id="logView" fx:controller="LogController" xmlns:fx="http://javafx.com/fxml" >
<TextArea text="-------- Log ---------" id="logTextArea" fx:id="logTextAreaFx"></TextArea>

<VBox minHeight="520" minWidth="1280" fx:controller="MainController" xmlns:fx="http://javafx.com/fxml">
<Button text="test Button" onAction="#handleChatConsoleButton"></Button>

public class ClientController {
private ClientView clientView;
@FXML private MainController mainController;
@FXML private LogController logController;
public ClientController(){
    this.clientView = new ClientView();
}
public void showView(){
    this.clientView.startView();
}
public void setText(String text){
    logController.handlelogTextAreaFX(null, "hello world");
}

public class GameMain {
static ClientController controller;
public static void main(String[] args){
    controller = new ClientController();
    controller.showView();
}

public class ClientView extends Application{
VBox root = new VBox();
VBox mainStage;
public void startView(){
    launch();
}
@Override
public void start(Stage primaryStage) throws Exception {
    Scene scene = new Scene(root);
    loadMainStage();
    loadLogStage();
    primaryStage.setScene(scene);
    primaryStage.show();
}


 @FXML Pane MainStage;
public void loadMainStage(){
    mainStage = new VBox();
    try {
        mainStage = FXMLLoader.load(getClass().getClassLoader().getResource("MainStage.fxml"));
    } catch (IOException e) {
        e.printStackTrace();
    }
    root.getChildren().add(mainStage);
}
@FXML VBox LogStage;
public void loadLogStage(){
    //ClientController controller = new ClientController();
    FXMLLoader loader = new FXMLLoader();
    Pane root = new Pane();
    Scene chatScene = new Scene(root);
    Stage stage = new Stage();
    stage.setTitle("Log View");
    stage.setScene(chatScene);
    stage.setResizable(true);
    try {
        //loader.setController(controller);
        LogStage = loader.load(getClass().getClassLoader().getResource("LogStage.fxml"));
    } catch (IOException e) {
        e.printStackTrace();
    }
    root.getChildren().add(LogStage);
    stage.show();
}