Glade GtkHeaderBar, add two children on both sides? - gtk

Question

I have searched existing questions, and the answer said change the pack type but the answer did not give details. I could add a button and change its pack type to "End", but if I try to add another child to set its pack type to "Start", I get the following error message that it needs some sort of placeholder. How do I do that? How do I add, in the example below, add another button on the left side of the header bar?
<?xml version="1.0" encoding="UTF-8"?>
<!-- Generated with glade 3.40.0 -->
<interface>
<requires lib="gtk+" version="3.24"/>
<object class="GtkWindow">
<property name="can-focus">False</property>
<property name="default-width">440</property>
<child>
<placeholder/>
</child>
<child type="titlebar">
<object class="GtkHeaderBar">
<property name="visible">True</property>
<property name="can-focus">False</property>
<property name="title" translatable="yes">Damn complex</property>
<property name="subtitle" translatable="yes">why</property>
<property name="show-close-button">True</property>
<child>
<object class="GtkButton">
<property name="label" translatable="yes">button</property>
<property name="visible">True</property>
<property name="can-focus">True</property>
<property name="receives-default">True</property>
</object>
<packing>
<property name="pack-type">end</property>
</packing>
</child>
</object>
</child>
</object>
</interface>
PS: I could achieve that by adding children programmatically in C#, but I still have not found a way to do this within Glade.
private MainWindow(Builder builder) : base(builder.GetRawOwnedObject("MainWindow"))
{
builder.Autoconnect(this);
DeleteEvent += Window_DeleteEvent;
var btn1 = new Button() { Label = "Left"};
var btn2 = new Button() { Label = "Right" };
header1.PackStart(btn1);
header1.PackEnd(btn2);
header1.ShowAll();
}

Answer 1

I found a way. Change "Number of Items" to 2, and then add 2 buttons. Change each button's pack type.