I've always put the ribs on top. chicken on the bottom. I agree with Alan, don't want raw chicken dripping juices on other meats. I also wait to the put the chicken on while foiling my ribs( 3-2-1, but sometimes its more like 3-1.5-1.5) That gives me 3hrs cook on the chicken at 230-260. Chicken gets 2hrs at ~230 and an hour at a higher temp. I don't exceed 270 because I use sugar in my rib rub and don't want to risk it burning.
I raise the temp of my smoker in the last hour with goal of crusting up the ribs slightly and crisping the chicken skin. Also, the chicken being on the bottom means I can get my ribs of quickly when needed because sometimes the chicken takes a little longer than 3hrs....