Question

How to implement such functionality that a button is enabled only if two lineEdit’s are filled with text?

Answer 1

You want to monitor both line edits for changes:

connect(lineEdit1, SIGNAL(textChanged(const QString&)), SLOT(checkShouldEnableButton()));
connect(lineEdit2, SIGNAL(textChanged(const QString&)), SLOT(checkShouldEnableButton()));

And then you need to enable/disable the button when text is present in both:

void YourWidget::checkShouldEnableButton() {
    button->setEnabled(
        !lineEdit1->text().isEmpty() && !lineEdit2->text().isEmpty()
        );
}

If you only care about user edits, you can use the textEdited(const QString&) signal instead of the textChanged signal.