I also doubt that two FMBs could do it. I have no doubt that the brackets themselves could take the strain, but the weak point here would be the MDF itself into which the brackets must be screwed. The M80s are 17" deep and 57 pounds each. Most of the weight is concentrated towards the front of the speaker, which means its center of gravity is pretty far away from the wall.
What this all boils down to is a tremendous force being placed on the mount points. Look at this diagram (that I actually drew to scale):
Gravity pulls straight down on the speaker at the center of gravity (the green crosshairs). Imagine the torque all the screws would have to endure in order to keep the speaker from tearing away.
While the FMBs are very sturdy, I doubt they were designed to hold that kind of load. Try holding a 40 pound weight close to your body, and then try holding it at arms length. Feel the difference?
