If you want to place a sandbag ontop of another one, then place two sandbags in the editor, one over the other and type the following code into one of the sandbags init field
this setpos [(getpos this select 0), (getpos this select 1), X]
replace "X" with what ever height you want (1, -11, 99138, etc...)
The same thing can be used to raise machine guns so that they are the same height as the window.