Stairway over Stairway

[Mainer]

I've got one stairway on top of another. When I try to move the narrower upper one so it is more directly over the botom one, it falls down onto the lower one. The upper one has a vertical rise of 108" and the lower one 120" (15 treads vs. 17 treads) so the bottom one is longer but only by 3". I can't slide the top one back even 6" and it falls down below. I don't think the upper one being narrower would affect it?

Any suggestions on how I can slide the top one back?