I like to expand my options in case I need to add anything additional in there.It's also very difficult to do for stacked slots like that. On a Dell/Lenovo/HP/whatever they can pull it off because they'll design a custom 2-slot riser board. For most inexpensive 1U cases, it's just a simple single-slot right angle riser board to get the PCIe slot to horizontal. It'd be hard/impossible to do two of them, because the first card would be in the way of the second one.
Can I ask *why* you feel the need to have a 1U system with two PCIe slots in it?