Asingle brick falls with acceleration g. the reason a double brick falls with the same acceleration is

EVERYTHING falls with the same acceleration near the earth's surface.

I thought about this for many years.  I finally understood WHY, and here's the way I like to explain it:

-- It takes MORE FORCE to accelerate a larger mass, and LESS FORCE to accelerate a smaller mass.

-- But guess what !  Gravity exerts MORE FORCE on a larger mass, and LESS FORCE on a smaller mass.

So everything averages out, and the acceleration is always the same, no matter what's falling.

To say it just a little bit differently:

-- A larger mass NEEDS more force to accelerate, but it GETS more force from gravity.

-- A smaller mass NEEDS less force to accelerate, but it GETS less force from gravity.

So everything averages out, and the acceleration is always the same, no matter what's falling.