I have one of the wheeled carts and I've never thought of putting them under the table. Then again, they take up a lot of space so I wouldn't carry one, anyway.
can't visualize the risers? hmmmm.
Imagine a table, long and rectangular. Pushed against the wall. Okay?
now imagine a board, about 10" wide, with legs on it, so that it forms a little table of its own. Put that on the table, Push it to the back.
now, from the front, you have a stepped/tiered table. You can display on both levels. From the back, t hough, you have a space where you can put boxes with restock items.
In this scenario the 'wall' is back of the table, where you'd be behind. I ask you to visualize the wall so that you see that the riser is to the back, and covered no more than 50% of the table top, width.
hope that helps. Using them sure has helped me.