I have a CB on Sweden for something I don't recall for the monent. But to use that I'll either have to wait until my kids are grown up to marry relatives of the danish and norwegian royal houses or wait until I have money enough to pay a whole bunch of mercenaries. If the relatives in Denmark and Norway become allies who will join a war on my side I could maybe use them and pay some mercenaries as well. Or I'll become a vassal of Sweden and hopefully get granted the title Duke of Västergötland. It could maybe be possible to use that title and a fabricated de jure CB to take Halland and Skåne from Denmark. Västergötland has de jure on Viken (Norway) so I could maybe use that too.
But I don't really know if I as the Duke of Västergötland am able to use a de jure CB on counties belonging to another independent country. That would probably mean a war between Sweden and the other country and not a war between two Dukes.