ABBYY will automatically separate the attachments into individual documents based on the information found on each page. However, if ABBYY has incorrectly separated the documents and if you would like to combine them into a single document, you can do this through the Batch Content Panel. 

You can simply drag and drop the pages from one document to another to merge them.

Alternatively, you can also select the documents and then click on the Merge button.

Once done, click on Apply Changes to save the changes.