Surely the ones sandwiched in the middle retain more heat than if they were standing separate to eachother...
True.
Heat dissipation depends on surface/area.
Stacking them like that, you lose the surface of each top/bottom.
Just look at the Avalons as if they were individual fins of a single heatsink.
You want this fins apart, not touching each other.
If this is the same idea as spacing on a server rack... wouldn't devices that have front-to-back cooling negatively effect cooling by spacing them apart?
EDIT: I assume, in this setup, you would also lose airflow to the top units by adding spacing... I'd leave them stacked.