Question

Can two XAML files have one .cs file?

Answer 1

Yes, because the backing source file only contains a partial class we can move it to any place we want. However this is not desired practice.

UserControl1:

<UserControl x:Class="WpfApplication1.UserControl1"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
    <TextBlock Text="UserControl1"/>
</UserControl>

UserControl2:

<UserControl x:Class="WpfApplication1.UserControl2"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
    <TextBlock Text="UserControl2"/>
</UserControl>

Source:

namespace WpfApplication1
{
    public partial class UserControl1 : UserControl
    {
        public UserControl1()
        {
            InitializeComponent();
        }
    }

    public partial class UserControl2 : UserControl
    {
        public UserControl2()
        {
            InitializeComponent();
        }
    }
}