This has to be combined with tabs feature of Kwin. It happens that, when you group 2 windows (to form tabs), there're still 2 entries in the taskbar. You do have an option to group them, but you have to click twice to switch to the Window. I suggest a single taskbar entry for the grouped window, that way, you have to click only once to hilight the whole group, then you can switch windows via the tabs.
*** This bug has been marked as a duplicate of bug 224690 ***