You can split a large user interface by defining UserControls.
Right-click on the solution tree, choose Add->New Item... then User Control. You can design this in the normal way.
You can then reference your usercontrol in XAML using a namespace declaration. Let's say you want to include your UserControl in a Window. In the following example I've added a UserControl named "Foo" to the namespace "YourCompany.Controls":
<Window x:Class="YourCompany.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:Controls="clr-namespace:YourCompany.Controls">
<Controls:Foo ... />
For your specific example, you would make use of your usercontrol in a combobox by defining a DataTemplate that displayed the data within your usercontrol.