Please see my comment to the question.
If your user controls basically works, I guess your problem is not user control or Forms interop,
but general XML design and layout. You cannot use "the same user control", you can use "another user control of the same type", if you need to. The problem is where you put it.
First of all, you need a container to put more controls. In the simplest case, the immediate logical of you window should be some
panel, that is, a UI element derived from the class
System.Windows.Controls.Panel, such as
System.Windows.Controls.Grid:
https://msdn.microsoft.com/en-us/library/system.windows.controls.panel%28v=vs.110%29.aspx#inheritanceContinued[
^],
https://msdn.microsoft.com/en-us/library/system.windows.controls.grid%28v=vs.110%29.aspx[
^] (for example).
Panels allow you to have multiple UI element as children. Choose what you want to use and read on the usage.
And simply add the similar XAML declaration in another cell of such panel or a different panel.
For basic general understanding: some WPF elements can have one child element in a
logical tree, via the
Content property, and other can has several children. For further detail, please read and understand it:
https://msdn.microsoft.com/en-us/library/vstudio/bb613556%28v=vs.90%29.aspx[
^],
https://msdn.microsoft.com/en-us/library/ms753391%28v=vs.110%29.aspx[
^].
Another your problem is sizes and layout. Basically, you either choose the size of the window, and then all your content should occupy available space according to layout, which can be pretty sophisticated. For example
Grid can define some column width as automatic, depending on the size of content, and other could divide the remaining rooms in proportions you define. Different UI element have different layout features like that. Alternatively, you can leave default size of the window (NaN, skip the attributes in XAML at all), and then the size of content will define the size of the window. If you define both size of, say, your user control and parent (say, window) size, it may conflict. Design your layout to automatically define sizes. For the size defined by window's contents, please see:
https://msdn.microsoft.com/en-us/library/system.windows.window.sizetocontent%28v=vs.110%29.aspx[
^],
https://msdn.microsoft.com/en-us/library/system.windows.sizetocontent%28v=vs.110%29.aspx[
^].
Note that this decision is done separately for vertical and horizontal layout.
See also:
https://msdn.microsoft.com/en-us/library/ms745058%28v=vs.110%29.aspx[
^],
http://wpftutorial.net/LayoutProperties.html[
^].
—SA
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