Trays are a rare luxury at most Pre-check lines and I am not about to stack them up for two reasons.
#1 I've personally witnessed a situation where a passenger put their bin on top of another one, only problem was that the other passenger didn't see that happen and started looking for their now buried cellphone which they expected to see in the top bin. This started a long charade of mutterings about a stolen phone and TSA goons running around trying to find it.... It was solved eventually.
#2 it's not my job! Seriously, we need to give
Tub
Stacking
Agents something to do, I mean, there are
Thousands of them
Standing
Around looking furtive.