That is simple!
One on each rack!
If you are only smoking one thing, then it doesn't really matter unless you have noticed a difference between the two racks that would affect the smoking. However, if you are cooking things that will be done at different times or will need more tending to while they cook or you don't want drippings from one down on to the other, then you may want to consider food placement. Me? I put the slower foods on the bottom so that I can get the faster cooking foods off of the top with out having to shuffle the generally bigger foods out of the way. Also for the most part the top rack smaller and faster foods tend to be the "daintier" stuff that I wouldn't want drippings on (ABTs , Mushrooms, small fish fillets and the likes) Some folks would think that the fish dripping on to the lower meat would be undesirable, and I agree, so I tend to cook it with foil under it>