When a assembly is not found, when using the standard probing method, an assembly resolve event is fired, allowing to load the assembly from a different file system path:

public void Test()
{
    AppDomain currentDomain = AppDomain.CurrentDomain;
    currentDomain.AssemblyResolve += new ResolveEventHandler(LoadFromSameFolder);
}
public static Assembly LoadFromSameFolder(object sender, ResolveEventArgs args)
{
    string folderPath = Path.GetDirectoryName(Assembly.GetExecutingAssembly().Location);
    string assemblyPath = Path.Combine(folderPath, string.Format("{0}.dll", new AssemblyName(args.Name).Name));
    if (!File.Exists(assemblyPath))
    {
        assemblyPath = Path.Combine(folderPath, string.Format("{0}.exe", new AssemblyName(args.Name).Name));
        if (!File.Exists(assemblyPath))
        {
            return null;
        }
    }
    Assembly assembly = Assembly.LoadFrom(assemblyPath);
    return assembly;
}