To connect several documents, first click on the document overview in the Webapp. After that you select the corresponding documents by clicking on the ticks in the upper right corner of the documents (appear when you move with the mouse over it). Then the merging symbol appears in the upper bar:
Once you have clicked on it, a new window will pop up, specifying which is the main document. The other document is then attached accordingly and it's metadata (title, sender, date) is discarded. Thus only the data of the main document remain.
If you click on merge, you have successfully merged your documents.