I'm a noob. But, what I'd probably do is start the cook about 18 - 20 hours before you plan to eat. It depends on if your guests won't mind waiting. If the meat is done in 12 hours or so, you can always let it rest in a cooler. It will stay warm for hours. I'd put the butt on the bottom rack like was mentioned. About 5 hours before the butt is done, I'd put the ribs on the top rack. Guessing 5 hrs before the butt is done would be the hard part. If you guess right, both should be ready about the same time. The last butt I cooked went about 17 hours. The last ribs I did cooked about 5 hrs at 250.
Last time I cooked the butt, I had it ready a day or two ahead of time. It's harder for me to predict how long it will take to cook. The ribs are pretty predictable. So, if you are like me and enjoy the atmosphere that cooking creates, you could do you butt a day or two ahead of time and then cook the ribs on the day your guests will be served. It will be less stressful, for sure.