When I learned to bag groceries, lo, so many years ago, it was taught that to the best of your ability, you try to make every bag look approximately like this:
You do not put all the heavy things in one bag, and all the light things in another:
This is why:
The same goes for this:
Not that I'm saying everything needs to be homogenized here. While the weight distribution of your bags really should be about equal, the temperature is a different matter altogether.
Let's look at this informative diagram of the direction heat moves:
Notice that heat moves away from its source and toward areas of no heat (or less heat).
This is why we separate warm items from cold items:
OK, thanks for letting me get that off my chest finally. For my next lecture....