Solution of the Support request:
Following example code, shows how to clip a new document to a existing document in a file cabinet. The docid was used to fetch the documents. This procedure can be advanced to an unlimited amount of documents, by using a loop.
        public static void ClipDocuments()
        {
            // Get id of existing document from filecabinet
            FileCabinet fc = organization.GetFileCabinetsFromFilecabinetsRelation().FileCabinet.First(f => f.Id == fcid);
            int doc1 = fc.GetDocumentsResult().Items.First(d => d.Id == docid).Id;
            // Upload second doc and fetch docid
            int doc2 = EasyUploadToFileCabinet(fc);
            // ClipDocuments documents
            List<int> docids = new List<int>()
            {
                doc1,
                doc2,
            };
            Document mergedDocument = fc.PutToContentMergeOperationRelationForDocument
                (
                    new ContentMergeOperationInfo()
                    {
                        Documents = docids,
                        Operation = ContentMergeOperation.Clip,
                        Force = true,
                    }
                );
        }
        private static int EasyUploadToFileCabinet(FileCabinet fc)
        {
            var document = fc.EasyUploadSingleDocument(
                new System.IO.FileInfo("C:/example.pdf"));
            return document.Id;
        }