multi_surface.pde

PGraphics pg;
PSurface surf;

void settings() {
  size(320, 240);    
}
  
void setup() {
  pg = createGraphics(500, 200);
  surf = pg.createSurface();
  surf.initFrame(this);
  surf.placeWindow(new int[] {100, 100}, null);
  surf.setVisible(true);
  surf.startThread();
}

void draw() {
  background(255, 0, 0);
  fill(255);
  rect(10, 10, frameCount, 10);
}

void mousePressed(){
  println("mousePressed"); 
}

Actually, previous code is wrong, because you still need a PApplet to drive the rendering loop of the secondary surface. This is a better solution.