Use PreviewMouseDoubleClick
event (msdn):
XAML:
<ToggleButton x:Name="tButton" Height="30" Content="MyButton"
PreviewMouseDoubleClick="tButton_PreviewMouseDoubleClick"
/>
Code-behind:
private void tButton_PreviewMouseDoubleClick(object sender, MouseButtonEventArgs e)
{
tButton.IsChecked = !tButton.IsChecked.Value;
e.Handled = true;
//...
}