Просто разделите флаги запятыми.
Test.ascx
<%@ Control Language="C#" AutoEventWireup="true" CodeFile="Test.ascx.cs" Inherits="Test" %>
<asp:Label ID="lblTest" runat="server"></asp:Label>
Test.ascx.cs
public partial class Test : System.Web.UI.UserControl
{
public TestEnum MyProperty
{
//coalesce was done to be lazy. sorry. haha.
get { return (TestEnum)(ViewState["te"] ?? TestEnum.One); }
set { ViewState["te"] = value; }
}
protected void Page_Load(object sender, EventArgs e)
{
lblTest.Text = MyProperty.ToString();
}
}
[Flags]
public enum TestEnum : int
{
One = 1,
Two = 2,
Four = 4,
Eight = 8
}
Test.aspx
<%@ Page Language="C#" AutoEventWireup="true" CodeFile="Test.aspx.cs" Inherits="Test" %>
<%@ Register Src="~/Test.ascx" TagPrefix="test" TagName="Test" %>
<form id="form1" runat="server">
<test:Test ID="test" runat="server" MyProperty="Four,Eight" />
</form>