Oracle Form : open a new form but the original one is closed

very new to oracle form.  I am in EBS 11.5.10.2.  Create a custom form with 2 windows ( also 2 Canvases.  Each window attach 1 Canvas.  They are like first  window .  The second window is to offer detail information ) My goal is to .. after click "open" button in the first Window/ Canvas, it will open up the second window/Canvas.  It is working so far.  But after the second window is open, the first one is gone.  They are related.  the second window is the detail of the first window.  How to keep the first window remain open after the second one is opened?

I use the apps_custom.Open_window() in the when-button-pressed trigger.  Here is the code.  What did I miss