Previous to our upgrade to 7.6.5, we could do the following with the old multinode treepicker using the content service:

if (ApplicationContext.IsReady) {
    var contentService = ApplicationContext.Current.Services.ContentService;
    var pageResults = contentService.GetChildren(1220);

    if (pageResults != null && pageResults.Any()) {
        foreach (var pageItem in pageResults) {
            string pageName = pageItem.Name.ToLower().Trim();
            if (pageName.StartsWith("thing")) {
                pageItem.SetValue("Brand", 2495);
            } else {
                pageItem.SetValue("Brand", "2276,2495");
            }

            contentService.SaveAndPublishWithStatus(pageItem);
        }
    }
}

With the new version, we have to use a udi... and I don't know how to set multiple nodes on this property. Example:

if (ApplicationContext.IsReady) {
    var contentService = ApplicationContext.Current.Services.ContentService;

    var thing1 = contentService.GetById(2495).GetUdi();
    var thing2 = contentService.GetById(2276).GetUdi();

    var pageResults = contentService.GetChildren(1220);

    if (pageResults != null && pageResults.Any()) {
        foreach (var pageItem in pageResults) {
            string pageName = pageItem.Name.ToLower().Trim();
            if (pageName.StartsWith("thing")) {
                pageItem.SetValue("Brand", thing1.ToString());
            } else {
                // how to get thing1 and thing2 together???
                pageItem.SetValue("Brand", thing2.ToString());
            }

            contentService.SaveAndPublishWithStatus(pageItem);
        }
    }
}

Why the change and why doesn't the previous version of this code work anymore?