This saddle would not work on a broad, flat back - these panels would only make contact on the outside edge, and would create pressure points and very likely make the horse back sore.
However, there are exceptions to every rule. Here's a photo of a plain panel with a very flat bearing surface (it was actually too flat for our saddle buck!):
The channel between the panels must be wide enough so that the panels don't impinge on the spinous process. Here's a picture of a rather ancient saddle (the tree in this saddle is twisted, which is why it's sitting crooked on our model) with inadequate channel width (red highlights the panel's bearing surfaces, green shows channel width, and orange shows the width of the horse's spine):
While the channel must be wide enough to clear the spine, it can't be too wide. Too much channel width can allow the saddle to drop down onto the spine, can create pressure points on the inside edge of the panel, or can allow the cantle area to shift side-to-side.