They are separate.
I took the notification off of the parts/assemblies. If the model changes, the drawing will change. You should only need that notification.
If the drawing changes because of a note/added dims, the model might not change. So there's really no point in doubling up on model side of notifications.
Random question. Are your drawings going through a different workflow? Or different transitions in the same workflow?
If all the documents are passing exactly through the same states and transitions, they should all be lumped into a single notification. However, if you have a separate transition for drawings (doing different actions) or maybe a different workflow all together, then it will push separate notifications.
If everything is going through the same states and transitions (going from Pending Approval to Approved using a single "Approve" transition) and you are still getting separate notifications, then I would recommend discussing this with your reseller as a bug.