I did a 22lb spatch turkey last year and it turned out great. My notes say I did 250F for 4.5 hours to get it to 160F internal. For your smaller ones I'd guess at least an hour less. I also opened up the smoker and basted it with melted herb butter about 2-2.5 hours in, cause butter makes everything better.
I'd just put them one over another on the two most centered racks in the smoker. Might need to leave an empty slot in between for enough clearance. If they're roughly the same size/weight you probably only need to temp probe one of them. If you want to get fancy you could swap the racks halfway through, but probably not necessary.