如何在WPF中组合两个显示？

Question

我有两个显示器。一个驻留在UserControl中，另一个是Rectangle。我将Rectangle作为参数传递给UserControl方法。现在在我的ViewModel方法中，我想将此Rectangle与UserControl显示组合在一起。如何实现这种组合显示？

ViewModel Code - 

   private UserControl CreateCombination(Rectangle rect)
        {
            //This UserControl contains one its display (basically a combination of few controls)
           CombinedUC combination = new CombinedUC();

            // I want to write some code here so that I can place combination object just above my rectangle (rect).

            return combination;
         }