Rotating simultaneously in the X and Y axis. Transformation functions such as rotate() are additive. Successively calling rotate(1.0) and rotate(2.0) is equivalent to calling rotate(3.0). 

a = 0.0


def setup():
    size(640, 360, P3D)
    global rSize
    rSize = width / 6
    noStroke()
    fill(204, 204)


def draw():
    global a
    background(126)

    a += 0.005
    if a > TWO_PI:
        a = 0.0

    translate(width / 2, height / 2)

    rotateX(a)
    rotateY(a * 2.0)
    fill(255)
    rect(-rSize, -rSize, rSize * 2, rSize * 2)

    rotateX(a * 1.001)
    rotateY(a * 2.002)
    fill(0)
    rect(-rSize, -rSize, rSize * 2, rSize * 2)