Geez, some people are never satisfied...
I don't know; try experimenting with it. You'll have to create a second set of image masks set to the desired location/size of the second spot, and then a second merge shader and displacement shader to control the height of the second flat spot and merge it into the terrain. You'll have to experiment to find the easiest way to actually carve the second chunk from the ter before mergng the flat spot(s).