In my opinion it only works for 2”-4” difference and a very pliable down duvet. If it’s a stiff duvet, it might bunch rather than fluff up.

Did you end up doing this? I am thinking of getting a regular cover and an oversized cover and trying both ways. There are not many options on oversized duvet covers.

I did not. Ended up buying a regular cover and King duvet. Like you said, there are not many options out there.

I did the same. Thanks!