Hi gnetsys,
Based on your description, do the two child windows are both modal dialog? And how do you place a Save and Close buttons? I create a custom modal dialog with this blog. In this blog, it just create one modal dialog. I modify it with add a button named "AddAnotherModalDialog"
which used to pop up the second modal dialog. And on the second modal dialog, I just put one button which used to close current dialog.

Then double click the "Cancle" or "Second Close" button on design screen and add the close code in the button's _Execute method as below. Please notice the parameter in the CloseModalWindow method, it is the name of the modal window which
need to be close.
partial void EditDialogCancle_Execute()
{
// Write your code here.
this.CloseModalWindow("CustomEditDialog");
}
partial void SecondClose_Execute()
{
// Write your code here.
this.CloseModalWindow("Group");
}
Best Regards,
Weiwei