I have those same shelves. The fold able ones right?

I'd leave em empty until they get secured to the wall.
Or at least the top section of each shelf, so they can get to the cross bar for screwing to the wall.

Mine isn't stacked {just 1}.
But if I did stack some, I would have each section screwed to the wall studs - it should be a simple job for someone with a electric drill/screwdriver, screws -the right size- and knowing how to find the studs in the wall.

the board that goes side to side across the back will give plenty of leeway as far as being able to hit the stud boards inside the wall. usually they are 16" on center {spacing} unless the house is older like mine

do be aware that wires may be inside the wall in certain areas and you don't want to hit those places {wall switches, outlets, overhead lights}
newer homes will have a metal protector but just something to think about..

And I would still keep the heavier books/ objects on the lower shelves just for safety.