That's the great part about having the multiple levels of grates on the KK. You can start the butts hours before you need to do the ribs and just put the ribs on the upper grate when the time comes. If you want to do more than a couple of racks, then get a vertical rib rack to maximizing the cooking space. I'd put the upper grate on when you put the butts on the main grate and they'll be preheated when you add the ribs. If you put the upper grate on at the beginning, when you are heat soaking the KK, then you have to juggle hot grates to put the butts on. If you wait, then you avoid this minor inconvenience, without sacrificing cooking times.
As far as actual timing, it depends on the temp you're cooking at, but at the basic 250F low & slow, a full bone-in pork butt/shoulder is going to take roughly 14 - 18 hours (depending on size and target internal meat temp), where ribs will be done in 3 - 5 hours, typically, depending upon what type of ribs - baby backs take less time than St. Louis.
Hope this helps a little, but the best advice is to just take notes as you do your cooks and adjust accordingly.