of inertia for 2 identical beams side by side or stacked vertically with
no connection between the two with a beam of the same width but with
doubled depth.
For 2 beams side by side:
S = 2 beams * 1/6 * b *d^2 = 1/3 bd^2
I = 2 beams * 1/12 * b *d^3 = 1/6 bd^3
For a beam with doubled depth:
S = 1/6 * b * (2d)^2 = 2/3 bd^2 or twice of the parallel beams
I= 1/12 *b * (2d)^3 = 2/3 bd^3 or 4 times of the parallel beams