var masterDocs = await DownloadXmlDocAssetAsync(masterUrl, isPreRelease:false);
var nextDocs = await DownloadXmlDocAssetAsync(nextUrl, isPreRelease:true);
return masterDocs.AsEnumerable()
.Concat(nextDocs.AsEnumerable().Where(kvp => !masterDocs.ContainsKey(kvp.Key)))
.ToDictionary(kvp => kvp.Key, kvp => kvp.Value);