No, i'm in the same case. The pushbutton in the mainwindow just shows the second window (form1). The second window has its own pushbutton just like you.
How does you second window is created and shown?

Complete code :
mainwindow.c
Qt Code: 
  1. #include "mainwindow.h"
  2. #include "ui_mainwindow.h"
  3.  
  4. #include <form1.h>
  5.  
  6. MainWindow::MainWindow(QWidget *parent) :
  7.     QMainWindow(parent),
  8.     ui(new Ui::MainWindow)
  9. {
  10.     ui->setupUi(this);
  11.     connect(ui->pushButton, SIGNAL(clicked()), this, SLOT(openWindow()));
  12. }
  13.  
  14. MainWindow::~MainWindow()
  15. {
  16.     delete ui;
  17. }
  18.  
  19. void MainWindow::openWindow()
  20. {
  21.     Form1 *form = new Form1(this);
  22.     connect(form, SIGNAL(transfertCompleted(QString)), this, SLOT(updateLabel(QString)));
  23.     form->show();
  24. }
  25.  
  26. void MainWindow::updateLabel(QString text)
  27. {
  28.    ui->label->clear();
  29.    ui->label->setText(text);
  30. }
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form1.c
Qt Code: 
  1. #include "form1.h"
  2. #include "ui_form1.h"
  3.  
  4. Form1::Form1(QWidget *parent) :
  5.     QDialog(parent),
  6.     ui(new Ui::Form1)
  7. {
  8.     ui->setupUi(this);
  9. }
  10.  
  11. Form1::~Form1()
  12. {
  13.     delete ui;
  14. }
  15.  
  16. void Form1::on_pushButton_clicked()
  17. {
  18. //write data
  19. //i want when click it send data and also send text to mainform
  20.     emit(transfertCompleted("transfer complete"));
  21. }
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Qt Code: 
  1. #ifndef FORM1_H
  2. #define FORM1_H
  3.  
  4. #include <QDialog>
  5.  
  6. namespace Ui {
  7. class Form1;
  8. }
  9.  
  10. class Form1 : public QDialog
  11. {
  12.     Q_OBJECT
  13.  
  14. public:
  15.     explicit Form1(QWidget *parent = 0);
  16.     ~Form1();
  17. signals:
  18.     void transfertCompleted(QString);
  19.  
  20. private:    
  21.     Ui::Form1 *ui;
  22. private slots:
  23.     void on_pushButton_clicked();
  24. };
  25.  
  26. #endif // FORM1_H
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Qt Code: 
  1. #ifndef MAINWINDOW_H
  2. #define MAINWINDOW_H
  3.  
  4. #include <QMainWindow>
  5.  
  6. namespace Ui {
  7. class MainWindow;
  8. }
  9.  
  10. class MainWindow : public QMainWindow
  11. {
  12.     Q_OBJECT
  13.  
  14. public:
  15.     explicit MainWindow(QWidget *parent = 0);
  16.     ~MainWindow();
  17.  
  18. private:
  19.     Ui::MainWindow *ui;
  20. private slots:
  21.     void openWindow();
  22.     void updateLabel(QString text);
  23.  
  24. };
  25.  
  26. #endif // MAINWINDOW_H
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