Originally Posted by Boshizzle
The lower rack will not be as hot as the upper rack. Heat rises to the top. So, make sure you have a grate thermometer on both grates to properly monitor temps.
I did that for the inaugural cook on my 22.5 WSM and was very surprised to see the bottom grate reading higher.
I did not expect that at all. That's also the first cook where I instrumented both grates since I only just got an ET-732 to supplement my ET-73. (I did check all four probes in a pot of boiling water prior to the cook so I know it was not a calibration issue.) And I've been smoking on my 18.5 WSM for over three years so I'm familiar with the WSM.
Usually if you are cooking different things on the two grates, put the one that you expect to finish sooner on top. It will be easier to take it off without disturbing the other grate.
And no, that does not sound stupid - that's what I would expect because as others have noted, heat rises.