Aligning controls on both left and right side in a stack panel in WPF

Just do not use a StackPanel, StackPanels stack. They do, for obvious reasons, not allow alignment in the direction in which they stack. Use a Grid, with column definitions like so:

<Grid.ColumnDefinitions>
    <ColumnDefinition Width="Auto" />
    <ColumnDefinition Width="*" />
    <ColumnDefinition Width="Auto" />
</Grid.ColumnDefinitions>