KCalc app has great features like Scientific, Statiscal, Mathematical,Programming, etc. On the other hand. Kalk has a very simple, minimal UI/UX to do math. But the UI is better than KCalc. What do I suggest is merge both KCalc and Kalk. So, KCalc is the Backend and Kalk is the frontend. It will be very powerful and productive.
Unfortunately merging apps isn't really a thing. It's like saying that these two houses are each great in their own ways, and you should merge them into one house. The way you actually do this is to select one house, abandon the other, and improve the selected house in ways that add the elements of the other house while trying to avoid the ways that it wasn't ideal. It's the same with apps.