Thats the first time i've ever heard about this ... the closest i can remember is the very short-lived personal union between P-L Commonwealth and Sweden under Vasa
Never heard of it either.
In Umbrals scenario mentioning Denmark loosing the islands and Scania to Sweden seems a bit weird, especially as it would draw an Eastern focused Sweden more south.
Beside Denmark without the islands are really not Denmark I would guess it would be called something else especially if it takes substantial parts in the low counties, further more a Denmark without the islands would definitely go to war with Sweden over the islands drawing Denmark north/east.
A Denmark with the islands and also with Scania could theoretical ignore Sweden and focus just on Northern Germany and by that you gain 3 Scandinavian powers:
Norway's Island kingdom. Norway, Iceland, Greenland, Orkney, Shetland, Faeroe islands and maybe some more Scottish islands.
Sweden's Baltic Kingdom. Sweden, Gotland, Finland, Karelia, Estonia, Latvia and the area around St. Petersburg.
Denmark's North German Kingdom. Denmark, Scania, Schleswig-Holstein, Mecklenburg, Bremen, Oldenburg and Rügen.
For this to work you need to stop Denmark seeing the Swedish kings as usurpers, and therefor stop them trying to get the throne back, you also need to stop the Swedish kings from wanting to gain Scania, also they probably need to accept the Sound toll.
making Denmark even lose Scania (a core Danish territory) will draw Denmark into war with the occupier, practically making 3 Scandinavian powers
impossible.
Denmark focusing this much on Northern Germany would probably transform Denmark from a Scandinavian country to a "Northern Bavaria"