1)look at common in the properties bar for the 3d object.
2)choose your z elevation-how high it will be (on floor is 0)
3)lets just say u set it to x.
4)now if you scroll down further in the properties bar you'll see -behaviour, effects, container etc- keep scrolling till you see (again!)properties-its the same name as its bar.
5)there it will show some options including -z height.
6)z height it literally how much it is stretched or the depth
7)now if you make another 3d object with same properties but assign the z elevation to x times 2 it will be exactly on top of the first one.
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hope you understand this
here's a pic for clarification:
just edited this-i showed in pic z elevation 15 -above where it says "by radkampfwagen"-its meant to be z elevation 0(idk how i forgot)
Hope you understand
Radkampfwagen