ListItem.fxml

<?xml version="1.0" encoding="UTF-8"?>

<?import javafx.scene.control.*?>
<?import javafx.scene.layout.VBox?>

<VBox xmlns="http://javafx.com/javafx/8.0.60-ea" xmlns:fx="http://javafx.com/fxml/1" fx:controller="sample.Main$ListItemController">
    <children>
        <Label text="This is ListItem.fxml from ListItemController"/>
        <Label fx:id="lblName" text="Original Text from fxml"/>
    </children>
</VBox>

Main.java

package sample;

import javafx.application.Application;
import javafx.fxml.FXML;
import javafx.fxml.FXMLLoader;
import javafx.scene.Scene;
import javafx.scene.control.Label;
import javafx.scene.layout.AnchorPane;
import javafx.scene.layout.Pane;
import javafx.stage.Stage;

import java.io.IOException;

public class Main extends Application {


    public static void main(String[] args) {
        launch(args);
    }

    @Override
    public void start(Stage primaryStage) throws Exception {
        Pane root = FXMLLoader.load(getClass().getResource("MainView.fxml"));
        primaryStage.setScene(new Scene(root));
        primaryStage.show();
    }

    public static class MainViewController {
        @FXML
        private AnchorPane acContent;
        private Label myLabel;

        public void initialize() {
            FXMLLoader fxmlLoader = new FXMLLoader();
            try {
                fxmlLoader.load(getClass().getResource("ListItem.fxml").openStream());
            } catch (IOException e) {
                e.printStackTrace();
            }

            myLabel = (Label) fxmlLoader.getNamespace().get("lblName");
            ListItemController controller = fxmlLoader.getController(); //controller defined in  ListItem.fxml
            Pane root = fxmlLoader.getRoot();// this is root of ListItem.fxml
            acContent.getChildren().add(root);
        }

        @FXML
        public void btnHome() {
            myLabel.setText("btnHome pressed");
        }

        @FXML
        public void btnOne() {
            myLabel.setText("btnOne pressed");
        }

        @FXML
        public void btnTwo() {
            myLabel.setText("btnTwo pressed");
        }
    }

    public static class ListItemController {

    }
}

MainView.fxml

<?xml version="1.0" encoding="UTF-8"?>

<?import javafx.scene.control.*?>
<?import javafx.scene.layout.VBox?>

<?import javafx.scene.layout.AnchorPane?>
<?import javafx.scene.layout.BorderPane?>
<AnchorPane id="AnchorPane" prefHeight="400.0" prefWidth="600.0" xmlns="http://javafx.com/javafx/8" xmlns:fx="http://javafx.com/fxml/1" fx:controller="sample.Main$MainViewController">
    <children>
        <BorderPane prefHeight="400.0" prefWidth="600.0" AnchorPane.bottomAnchor="0.0" AnchorPane.leftAnchor="0.0" AnchorPane.rightAnchor="0.0" AnchorPane.topAnchor="0.0">
            <top>
                <AnchorPane prefHeight="78.0" prefWidth="600.0" BorderPane.alignment="CENTER">
                    <children>
                        <Label fx:id="lblName" layoutX="506.0" layoutY="32.0" text="Rifat" />
                    </children>
                </AnchorPane>
            </top>
            <left>
                <AnchorPane prefHeight="200.0" prefWidth="200.0" BorderPane.alignment="CENTER">
                    <children>
                        <Button fx:id="btnHome" layoutX="49.0" layoutY="39.0" mnemonicParsing="false" onAction="#btnHome" text="Home" />
                        <Button fx:id="btnOne" layoutX="36.0" layoutY="87.0" mnemonicParsing="false" onAction="#btnOne" text="Seen One" />
                        <Button fx:id="btnTwo" layoutX="36.0" layoutY="136.0" mnemonicParsing="false" onAction="#btnTwo" text="Seen Two" />
                    </children>
                </AnchorPane>
            </left>
            <center>
                <AnchorPane fx:id="acContent" prefHeight="200.0" prefWidth="200.0" BorderPane.alignment="CENTER" />
            </center>
        </BorderPane>
    </children>

</AnchorPane>