Well I've heard of STORING chicken above pork, but as far as how they get put in the cooker, what's the difference?
Personally, if were me, if I were starting both at the same time, I'd put the chicken on top because it will be coming off earlier than the pork.
If I wanted them both DONE at the same time (or close) I'd probably do the pork , take it off to rest and then put the chicken on (as aawa suggested above).
If I was going to put the pork on and put the chicken on later while the pork was still cooking, then yes....I'd put the chicken on the bottom so that the uncooked chicken wasn't dripping onto the pork. STILL THOUGH....if the chicken was put on and had time to fully cook by the time both pork and chicken came off, then again....what difference does it make? You have cooked pork juice dripping on chicken or fully cooked chicken juice dripping on pork. It's all food safe at that point. Just depends on whether or not you want the chicken to taste of pork or the pork to taste of chicken (neither of which will really matter or have any real affect IMO).