@KuwaretaMihile
It says they're in a "shared universe". This means that, say a character from one series lived in Africa, the same character with the same personality and experiences (though possibly altered depending on when set) is able to appear in this series, living in Africa (unless, ya know, they moved).
So the same MC doesn't have to appear, but they have the ability to appear. Never read the combat butler series, but apparently some characters have appeared relating to that series (e.g. the cook). For Tonikaku Kawaii, the two MCs from that series have a cameo in this series. They don't have to appear in the series though for it to be a shared universe.