public class BillingModel
{
    [Required,
    DisplayName("First Name")]
    public string FirstName { get; set; }

    [Required,
    DisplayName("Last Name")]
    public string LastName { get; set; }
}

public class CustomerModel
{
    [Required,
    DisplayName("Address")]
    public string Adress { get; set; }

    [Required,
    DisplayName("City")]
    public string City { get; set; }
}

public class OrderViewModel
{
    public BillingModel Billing { get; set; }
    public CustomerModel Customer { get; set; }
}

<input id="Business_FirstName" name="Business.FirstName" type="text" value="" />

<input id="Business_LastName" name="Business.LastName" type="text" value="" />

@Html.TextBoxFor(x => x.Business.FirstName)
@Html.TextBoxFor(x => x.Business.LastName)

<script src="@Url.Content("~/Scripts/jquery.validate.min.js")" type="text/javascript"></script>
<script src="@Url.Content("~/Scripts/jquery.validate.unobtrusive.min.js")" type="text/javascript"></script>

@Html.TextBoxFor(x => x.Business.FirstName)
@Html.ValidationMessageFor(x => x.Business.FirstName)

@Html.TextBoxFor(x => x.Business.LastName)
@Html.ValidationMessageFor(x => x.Business.LastName)

<add key="ClientValidationEnabled" value="true" />
<add key="UnobtrusiveJavaScriptEnabled" value="true" />