Add the folder Id to the 'queue folder Id' field on the delivery option.
Step 1: Navigate to Document Queue Delivery Option Step 2: Click green arrow on right side Step 3: Hit Edit Layout Step 4: Drag Queue Folder Id onto the page layout Step 5: Create a new folder, ensure both users have access to the folder Step 6: Copy the folder ID and paste it into the newly added field.
Also, confirm that the user has "Read" permissions to the Document object
When a queue folder Id is not specified, the document will be placed in the “My Personal Documents” folder of the user that generated the file. This means that the any other user that tries to merge the document will not be able to do so.