Connection of 2 different workflows

Dear Community,

I have a question regarding the connection of 2 different workflows. I built a workflow of 'Parts' and another one for 'Documents'. 'Parts' and 'Documents' are two different ItemTypes and are connectd through a relationship 'One Part can have many Documents'.

The Workflow of 'Parts' can be defined as follow:

The Workflow of Documents is represented as follow:

Now, what I was trying to achieve is that once the activity 'Revision' is carried out in the 'Parts' workflow the 'Document' workflow should be opened and tasks sent to my InBasket. This is because, I want my employees to check if my documents related to a part need to be reviewed and revised once the latter has been revised. I know that I could extend the Parts workflow, however, I want to avoid that. 

I tried assigning the Document Workflow as a Subflow of the Parts 'Revision' activity and implementing the promotion with the Relationship Parts-Document as the ItemType. I added the Document Lifecycle and Workflow in the Relationship ItemType and in the Document ItemType. However, nothing seemed to work.  (PS: the two workflows work fine independently)

How should I proceed?

I would be very thankful for any help.

Best regards,

Florian