Question

how do i set the multiple datasource to single dropdownlist?

 DrpPosition.ClearSelection();
        DrpPosition.DataSource = AdvertizeManager.GetList("", "", "", "");
        DrpPosition.DataTextField = "PNAME";
        DrpPosition.DataValueField = "ADID";
        DrpPosition.DataBind();
        DrpPosition.DataSource = CategoryManager.GetList("", "", "", "");
        DrpPosition.DataTextField = "NAME";
        DrpPosition.DataValueField = "CATID";
        DrpPosition.DataBind();

Answer 1

You can use LINQ to easily solve this:

        var advertizes = AdvertizeManager.GetList...
        var categories = CategoryManager.GetList....

        // create your annonymous type
        var customAdvertizes = from advertize in advertizes
                select new
                {
                    Value = advertize.ADID,
                    Text = advertize.PNAME
                };

        var finalQuery = customAdvertizes.Concat(
            from category in categories
            select new
            {
                Value = category.CATID,
                Text = category.NAME
            }
        );

And then just bind it to your DropDownList

    DrpPosition.DataSource = finalQuery;
    DrpPosition.DataTextField = "Text";
    DrpPosition.DataValueField = "Value";
    DrpPosition.DataBind();