Can I mirror backwards? I want to mirror a building across the Y-axis, but I want the mirrored side to be flipped along the X-axis (I.E. be flipped). Is there a way to do that?
The best way to do this is to make a  linked duplicate of your original object (ALT+D) and then rotate it 180 degrees on the z axis. They will technically be separate objects, unlike  a modifier, but editing one will change the other accordingly.