How to add multiple toolbox controls in xaml

Answer 1

Panels.

<StackPanel>
    <Image ... />
    <Label ... />
</StackPanel>

Also see <WrapPanel>, <DockPanel>, <Grid>, <Canvas>, etc.

You'll run into that error if you do something like:

<Window ... >
    <Image ... />
    <Label ... /> <!-- Won't work -->
</Window>

This is because many controls in WPF are ContentControls, like Window, and these sorts of controls can only have one child. To have more than one child, use a Panel.