It all depends on what temp(s) you're thinking of using and whether you are cooking and holding, cooking and cooling/reheating later, or cooking and serving.
I cook both at a similar temp (325-350). The ribs would take less time. I'd likely ccok the turkey till a bit more than half done--maybe 3/4 done, lift it and the grate off and put the ribs on the lower grate. Then I'd return the turkey and finish cooking it, remove and rest and finish the ribs, perhaps foiled, perhaps not, depending on how far along they were when the turkey was done.
Another option is simply to cook the ribs ahead of time, cool and fridge, then cook the turkey nearer to when you need it. When the turkey rests, the ribs can be reheated.
