In 1660, Denmark-Norway had a population of around 1.1 million. Sweden had a population of around 2.2-2.5 million.
Yes but that was after the loss of Scania and the harrowing of Denmark. If we look at before the war Denmark had 1,5 million and Sweden 2 millions, and if we goes back before the 30YW the two countries had the same population size.
So while a post-loss of Scania wank are possible (there was support for a union with Denmark in the 1730ties), I think before are significant easier.
Let's start one place Christian IV oldest Christian the Chosen Prince was pretty useless, his next son Frederik III was intelligent, but his personality was pretty much a political disaster, the third son seem to have been talented but he died to young for us to find out anything about him. So let's change the first son, let's say for some reason he's more talented and a good ruler.
Next in OTL Christian IV wasted a lot of money on setting up a colony in India, and his search for north west passage failed. Let's say that someone convince Christian IV that it's a better idea to set up settlement in North America and take a short cut to India over land. Denmark found a colony in North America, maybe they buy Canada from France, maybe they settle in New York, the important part are that Denmark have set up a precense here. Beside that Christian IV invest in some African trading forts.
Next Christian end up more tolerant toward Reformed and he allow Reformed refugees to settle in Denmark.
Christian also set up a agricultural academy in attempt to introduce new crops and agricultural methods to Denmark.
When he fall off his horse in the 1620ties and dies.
He leave his crown to a more competent heir, who as a young king decides to stay out of the 30YW.
The Swedes still intervene and the kings two brothers (who are German bishop-princes) fight on the Swedish side. Denmark also end up supporting Sweden after the engagement, The youngest Ulrik becomes engaged to Christina of Sweden. Gustav II Adolf survive long enough to see Ulrik and Christina married in 1640. The Danish king on the other hand as a childless king end up making the Danish estate recognised Frederik III as his heir. As such Frederik III the bishop and later duke of Bremen-Verden end up Danish king, while Ulrik bishop of Schwerin end up king of Sweden.
The 30YW end up much as OTL except that Denmark gain Bremen-Verden, Hamburg, Bremen and the right to raise customs on the Elbe and Weser, while Sweden get all of Pomerania plus Wismar, Schwerin and the right to raise customs on the Oder. With the same dynasty in power in both states, they decides that Denmark accept everything east of Lübeck as Swedish sphere of interest and Sweden accept that everything west of Mecklenburg as as Danish sphere of interest.
With this Sweden and Denmark can pretty much ignore each other, Sweden can get away with its war with Russia and Poland, while Denmark can focus west and south. Later we can see a dynastic union as the two kingdom will keep intermarry.