How many stackers should I use on Bullet Smoker

I've got the pleasure of owning a bullet smoker (ProQ Excel 20, a UK model) and am about to do my first brisket.  I've got two stacker grills available but will only need to have 1 grill in action -- should i set up my bullet smoker with just one of the stackers and the grill above the water bath, or with both stackers and grill on the top one?

I'm inclined to go with a single stacker as temperature control will be easiest as I'm reducing the surface area of the smoker, but at the same time worry that this puts the brisket too close to the fire / smoke.  Any thoughts?

Thanks