Hi Stefan,
I'm sorry if I have misled you with my previous answer.
Yes, you can achieve this behavior but you should use some layout to arrange the new text primitives. By default each element uses a Canvas layout to position its elements and when you insert new elements in its structure they'll be positioned absolutely in the element's space.
I have created a sample class which demonstrates this approach:
Public
Class MyListBoxItem
Inherits RadListBoxItem
Protected Overrides Sub CreateChildElements()
MyBase.CreateChildElements()
Dim layoutPanel As ImageAndTextLayoutPanel = CType(MyBase.Children(2), ImageAndTextLayoutPanel)
Dim stripPanel As StripLayoutPanel = CType(layoutPanel.Children(1), StripLayoutPanel)
Dim itemText As TextPrimitive = CType(stripPanel.Children(0), TextPrimitive)
Dim newStripPanel As StripLayoutPanel = New StripLayoutPanel()
newStripPanel.Orientation = Orientation.Horizontal
Dim redText As TextPrimitive = New TextPrimitive()
redText.ForeColor = Color.Red
redText.Text = " Red "
Dim blueText As TextPrimitive = New TextPrimitive()
blueText.ForeColor = Color.Blue
blueText.Text = " Blue "
stripPanel.Children.Remove(itemText)
newStripPanel.Children.AddRange(
New RadElement() { itemText, redText, blueText })
stripPanel.Children.Insert(0, newStripPanel)
End Sub
End ClassHere's a screenshot of a listbox which takes advantage of this new listbox item (I have set empty string for the second item's description text):

You can easily enhance this class and expose the new primitives' Text properties as properties of the MyListBoxItem class.
Please, let me know if this works fine for you.
Best wishes,
Chris
the
telerik team