i'm fairly certain the lower flat "bar" is simply a support/mounting strut.

i'm also fairly certain you want to use the upper round bar for the panniers.

lookit the pitcher! the right pannier bottom is a'hangin' well below the lower bar.

pannier construction will usually have a flimsy rear support, thin plastic sheet,
which requires the solid rack tubing to maintain its shape. without a support
behind it, they tend to bend in the direction of least resistance.

the right side bag will likely be sucked into the spokes.

if you're "lucky", only the bottom of the bag will be ripped open..............