ok, spostando il tutto dentro una classe in quel package funziona.
codice:
package com.mp.fxml;

import javafx.fxml.FXMLLoader;
import javafx.scene.Parent;
import javafx.scene.Scene;
import javafx.stage.Stage;

import java.io.IOException;

public class FXMLUtils {
    public static void openWindow(String fileFxml, String title, int width, int height, boolean resiz) throws IOException, ClassNotFoundException {
        Parent root = FXMLLoader.load(FXMLUtils.class.getResource(fileFxml));
        Stage stage = new Stage();
        stage.setTitle(title);
        stage.setScene(new Scene(root, width, height));
        stage.setResizable(resiz);
        stage.show();
    }
}
da richiamare così:
codice:
package com.mp.testfx;

import com.mp.fxml.FXMLUtils;
import javafx.fxml.FXML;

import java.io.IOException;

public class ControllerUno {

    @FXML
private void onBtnClicked() {
        try {
            FXMLUtils.openWindow("due.fxml", "due", 500, 500, false);
        } catch (IOException | ClassNotFoundException e) {
            System.out.println(e.getMessage());
        }
    }
}
grazie!!