SusanSusan, When I use my slidelock ruler, I don't take away the marked ruler, I simply lay the slidelock on top of the other one and it works just fine for me that way.
I cut a lot of strips using the slotted rulers so it is much easier for me to keep the slots in place and simply move the slidelock along as I cut. The slidelock holds both the slotted ruler and the fabric in place just fine!!