Agreed, when it comes to big financial decisions for sure as they have to approve the $$$ portion of that transaction.
However I would imagine Leafs MLSE ownership has given Shanahan full autonomy on the day to day running of the Leafs which includes spending to the cap ceiling and hiring and firing anyone he wants to positions below him.
Shanny has the authority to fire Babcock the coach he hired though, whereas normally a GM gets to pick his own coach and fire them as well. I don't believe in Leafs structure with Dubas as GM he could fire Babcock, unless he first gets Shanahan's approval to do so.
I also doubt that Dubas would go to Leafs Ownership and say I want to sign Tavares for 7X $10 mil as a normal GM, as he again would need Shanahan to be onboard first with the signing and then Shanny the one likely to go the MLSE BOD for final $$$ approval on that type of long term commitment.
So I see it more as Shanahan is the Leafs real GM for all intents and purposes and Dubas is assistant GM in his roles and responsibilities.