Maybe - but that's still going to allow for some awkward problems. Say that the King of France declares war on the King of Asturias. The Holy Roman Emperor holds a duchy in each, and as such is obliged to support his liege in both the offensive and defensive wars. Make it more complex, and say it's a de jure claim by France on a French county held by Asturias. Who is the HRE forced to support? What happens with the (presently automatic) making of all your troops hostile to all enemy troops. Bear in mind that a vassal doesn't get a call to arms, he's just automatically involved with his overlord's wars.
If it was going to be done at all, it would have to be done on the basis of possessing or inheriting a previous vassal contract, not on the basis of which de jure Duchy, Kingdom or Empire the land was part of. The King of England owed fealty to the King of France for Normandy because William was Duke of Normandy (and, thus, a vassal to the French king) before he was King of England, and also IIRC inherited the Duchy of Aquitaine from a parent (and, thus, remained a de jure vassal of France) rather than taking it by force. If the HRE had taken de jure parts of Asturias and France, it would own them in its own right rather than as a vassal, so it wouldn't need to be called-in to either war.
Still, whilst I do agree in principle that the vassal contract should be attached to the title rather than to the individual (and, thus, that it should be possible for someone to have two different lieges or to have a title they hold independently as well as another they owe fealty for), I think that it would be challenging to implement such a feature, and would almost certainly require a significant change to the underlying game engine, meaning that such a change is going to have to wait for CK3. And, given the limitations, I think that what we have currently is a fairly decent approximation because, even if the English King did technically owe fealty to the French King, in practice that meant very little, aside from an excuse for the French King to revoke the title when he felt strong enough to do so (which, really, would be better represented by the ability to fight de jure wars over entire duchies rather than just counties).
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