Greg,
I have done this before and here is what I do.
First I need to tell you that, for ribs, my family and I prefer them to be falling-off-the-bone tender so I use the 3-2-1 or 2-2-1 (depending on whether I'm smoking spare of BB ribs) foil method. I also smoke butts until the SI has quit smoking and then remove them, place them in a pan and cover and finish either in the smoker or the oven. I have found (for me) these two methods are the most predictable as far as cook time goes. There are many on this forum that do not use these methods and I respect that. To each his own, as they say.
I put the ribs up top and the butt below and smoke them at 225-230 for 2-3 hours. I pull the butt, close the door and prepare it for the oven (I can give you details on how I do this if you want). I get the butt going in the oven and then go back to the ribs. I then finish the ribs in the SI using the method described above. If you time it right and depending on how long you want your butt to "rest" they can be ready to eat at about the same time, which usually makes mama happy.