TreelistEx souce query not working

Question

I have the content structure as follows:

sitecore
|_content
  |_Site1
    |_Home
      |_Articles
        |_Category
          |_article1
          |_article2
  |_Site2
    |_Home
      |_Rewards
        |_September
          |_Reward1
          |_Reward2

The user should be able to select articles & rewards in the treelist field. For the source query, I entered:

    datasource=/sitecore/content&includeitemsfordisplay=Site1,Home,Site2&
includetemplatesfordisplay=Articles Folder,Category Folder,Article,Rewards Folder,  
Month Folder,Reward&includetemplatesforselection=Article,Reward

It only shows the content node when trying to select.
When I keep only the includeitemsfordisplay parameter it shows as:

|_content
      |_Site1
        |_Home
      |_Site2
        |_Home

I followed @Swati Gupta's example but it shows a plain text field

Answer 1

If you want to give the option to select from multiple datasource, then you can create a custom control.

1. Create new item MultiDatasourceTreelist using /sitecore/templates/System/Templates/Template field type template in core DB in this location /sitecore/system/Field types/User Defined/MultiDatasourceTreelist

2. Create a class MultiRootTreeList

 public class MultiRootTreeList : TreeList
    {
        protected override void OnLoad(EventArgs args)
        {
            Assert.ArgumentNotNull(args, "args");
            base.OnLoad(args);

            if (!Sitecore.Context.ClientPage.IsEvent)
            {
                // find the existing TreeviewEx that the base OnLoad added, get a ref to its parent, and remove it from controls
                var existingTreeView = (TreeviewEx)WebUtil.FindControlOfType(this, typeof(TreeviewEx));
                var treeviewParent = existingTreeView.Parent;

                existingTreeView.Parent.Controls.Clear(); // remove stock treeviewex, we replace with multiroot

                // find the existing DataContext that the base OnLoad added, get a ref to its parent, and remove it from controls
                var dataContext = (DataContext)WebUtil.FindControlOfType(this, typeof(DataContext));
                var dataContextParent = dataContext.Parent;

                dataContextParent.Controls.Remove(dataContext); // remove stock datacontext, we parse our own

                // create our MultiRootTreeview to replace the TreeviewEx
                var impostor = new Sitecore.Web.UI.WebControls.MultiRootTreeview();
                impostor.ID = existingTreeView.ID;
                impostor.DblClick = existingTreeView.DblClick;
                impostor.Enabled = existingTreeView.Enabled;
                impostor.DisplayFieldName = existingTreeView.DisplayFieldName;

                // parse the data source and create appropriate data contexts out of it
                var dataContexts = ParseDataContexts(dataContext);

                impostor.DataContext = string.Join("|", dataContexts.Select(x => x.ID));
                foreach (var context in dataContexts) dataContextParent.Controls.Add(context);

                // inject our replaced control where the TreeviewEx originally was
                treeviewParent.Controls.Add(impostor);
            }
        }

        /// <summary>
        /// Parses multiple source roots into discrete data context controls (e.g. 'dataSource=/sitecore/content|/sitecore/media library')
        /// </summary>
        /// <param name="originalDataContext">The original data context the base control generated. We reuse some of its property values.</param>
        /// <returns></returns>
        protected virtual DataContext[] ParseDataContexts(DataContext originalDataContext)
        {
            return new ListString(DataSource).Select(x => CreateDataContext(originalDataContext, x)).ToArray();
        }

        /// <summary>
        /// Creates a DataContext control for a given Sitecore path data source
        /// </summary>
        protected virtual DataContext CreateDataContext(DataContext baseDataContext, string dataSource)
        {
            DataContext dataContext = new DataContext();
            dataContext.ID = GetUniqueID("D");
            dataContext.Filter = baseDataContext.Filter;
            dataContext.DataViewName = "Master";
            if (!string.IsNullOrEmpty(DatabaseName))
            {
                dataContext.Parameters = "databasename=" + DatabaseName;
            }
            dataContext.Root = dataSource;
            dataContext.Language = Language.Parse(ItemLanguage);

            return dataContext;
        }
    }

3. Add source in template Datasource=sitecore/_content/_Site1/_Home/_Articles|sitecore/_content/_Site1/_Home/_Rewards