Basically this is the "same" idea as force Nautilus to open in new tab instead in new window, just for Kubuntu.
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Dolphin has split view option which is even better than tabs as you can move your files much easier from folder to folder.
So when I have my Dolphin open and I want to open another Dolphin the first Dolphin should just split view to a folder that I opened with the second one.
If there is already a split view opened in Dolphin it should add a new tab instead, then split view, then tab again...

I hope you get the picture.
And if I close split view in both tabs they should be merged in one split view.