If you can take both the 18 and the 22, do. It will make your contest so much less of a headache. The ribs can then cook undisturbed on the 22, and the 18 has plenty of room for chicken. We usually do 16 thighs on an 18" WSM.
Also, if one of your long meats runs over, you can set up the 18 as an oven to finish it and not screw up your rib start time. The long meat should be done and resting long before you need the 18 for the chicken.
Don't practice until you get it right; practice until you can't get it wrong.
Don't overthink or undercook.
Rhythm 'n QUE