I recently upgraded from Ubuntu 16.04 to 18.04, and I don't like Gnome so I use Unity. However, when I Alt-tab,
both alt-tabs show at the same time
(sorry for poor image quality). The tab button works for both, and they seem to be working on top of each other, and so there's really no point in me using alt-tab.
I can provide any extra information if necessary, but I don't really know what's necessary.