The challenge, you mentioned to me that you're cutting them in 1/2, is you now have to fit 18 halves on three shelves. I think you'll run into a vertical problem with only 3 shelves. They'll also be very unstable that way, and you'll have to hold the door open and the heat will escape.
Only easy way is to go the rack method.
No issue with them touching, I just rotate the ribs so any touching get moved as you rotate.
You might also think of doing them in 2 batches. One batch, hold. 2nd batch in the smoker. When it's time to eat, throw the first batch in for reheating and server the 2nd batch first.