I have been using one of these for nearly a year. Because they are not that high (about 4 inches), I have been placing it on top of a larger foam cushion to gain the extra height to make it comfortable on the legs. Anyway, that arrangement has been less than satisfactory because its not very stable. So I ordered a second one and yes, it is now stable. Another reviewer said its like being sat on sand and I concur. This means the cushion shapes itself to the shape of your rear. This is a good thing because it is important to be stable and solidly positioned when sitting.
Some sites suggest that you only need one cushion unless you are tall. Well its not just about how tall you are, its about how long your legs are and how flexible your joints are. For those of us westerners who are not trained to sit cross legged on the floor from a young age and/or who are not very flexible in the joints, it is better to be higher off the floor. For reduced strain on joints, comfort and unrestricted circulation during longer sessions, your knees need to be well below your hips (thighs sloping downward). So if you feel you need a second cushion, then get one.