I was foam rolling the other day and wondered if it mattered whether or not you had both legs on the roller at once when rolling out you quads. Logically it makes sense that you are doubling the weight and doubling the area on which that force is acting. But this can also be thought of in terms of pressure, or force/surface area. Because putting a second leg under the foam roller while the force remains the same (as body weight and gravity are not changing) changes the surface area, the total amount of pressure felt is less.
Here is an example with rough estimates:
Surface area of one leg (the amount of thigh that would be of the roller) about 20 cm2 or 0.002m2
Force of thigh on roller: The thigh is estimated to be 12% of body weight in females (10.5% in males). We will assume the person weighs 130 pounds. 130 x 0.12 = 15.6 pounds or 0.45 kg
P= F/A = (0.45kg)(9.8 m/s2)/0.002m2
= 2205 Pa
P = 2(.45kg)(9.8 m/s2)/(0.002m2)2
= 2205 Pa
Through these calculations, you can see that the same amount of pressure is felt regardless of having one or both legs on the roller.