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In the palette there is a Frame option that creates a window inside another window, however, I do not know how to connect them through a button or if this is possible. The window created normally with the JFrame form if it appears in the package, the other does not (the one created when using the Frame section of the palette).
Regarding the question how to connect them, these are Jframes so in the button where you want to call them you only have to put:
NombreJframe.setVisible(true);
Probably because you have not defined a size in your main constructor just like this:
initComponents();
NombreJframe.setSize(WIDTH, WIDTH);
and inside you put the size you want.