I have faith in you, if you are adjusting one the other won't run away on you. Temp control is not precise you will get swings in temps all the time, even if you had both at the exact temp one will always finish first due to the difference in far and somewhat density.Monitoring two lit charcoal units at the same time is not that easy to maintain proper temperature control for some of us backyard BBQers
Harry soo says if you overload the 18 you can choke it out, he recommends the 22 for multiple full packers etc.It's easier to cook more on the 18.5" WSM then less on the 22.5" WSM.
I think Tim gets it right. You can squeeze more onto the 18 if needed on occasion.
I have my 18 set up with brackets for a third grate. Nice to have the extra horizontal space when doing ribs laid flat or wings. But most often I cook using only one grate.
Examples of the cooking capacity of the Weber Smokey Mountain Cooker. Capacity for chicken, pork ribs, pork butt, and brisket are shown in each cooker.www.virtualweberbullet.com