Rotate 1.

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).

 
float a = 0.0;
float rSize;  // rectangle size

void setup() {
  size(640, 360, P3D);
  rSize = width / 6;  
  noStroke();
  fill(204, 204);
}

void draw() {
  background(0);
  
  a += 0.005;
  if(a > TWO_PI) { 
    a = 0.0; 
  }
  
  translate(width/2, height/2);
  
  rotateX(a);
  rotateY(a * 2.0);
  rect(-rSize, -rSize, rSize*2, rSize*2);
  
  rotateX(a * 1.001);
  rotateY(a * 2.002);
  rect(-rSize, -rSize, rSize*2, rSize*2);

}