<%= Html.TextBox("amount", Model.amount, new { id = "Pamount" })%>
    <%=Html.ValidationMessage("Pamount", " ")%>
<%= Html.TextBox("amount", Model.amount, new { id = "Eamount" })%>
    <%=Html.ValidationMessage("Eamount", " ")%>

if (obj.amount.ToString() == "0")
            modalState.AddModelError("Pamount", "");

ModelState.AddModelError("amount", "");

<%= Html.TextBox("Foo.amount", Model.Foo.amount, [...]
<%= Html.TextBox("Bar.amount", Model.Bar.amount, [...]