I was wondering if I can have 2 controls in a horizontal-oriented StackPanel so that the right item should be docked to the right side of the StackPanel.
I tried the
You can achieve this with a DockPanel:
Left
The difference is that a StackPanel will arrange child elements into single line (either vertical or horizontally) whereas a DockPanel defines an area where you can arrange child elements either horizontally or vertically, relative to each other (the Dock property changes the position of an element relative to other elements within the same container. Alignment properties, such as HorizontalAlignment, change the position of an element relative to its parent element).
Update
As pointed out in the comments you can also use the FlowDirection property of a StackPanel. See @D_Bester's answer.