If you want to stop the reflow of measures from the first stave, then lock it. In order to move the second stave to the bottom of the page, adjust the top system margin, or the distance between systems, to an appropriate amount.

I suspect my solution may not be appropriate, in which case please give us a little more information. I'm guessing you want to create some kind of instructional method book, or some such.

My first thought (given the info you have provided) would be for you to create a two-stave system. Put the notes, etc. you want to go on the top on the top staff; then starting with the next measure, put the information you wanted to go on the bottom staff.

Force a system break between the two...

...and hide the unused staff in each system. You can then drag the second system where you want on the page.