Take a look at the ControlWindowCanvas class, and the following example:

perfect. wondering why i didnt find that example.
Thx

hmm, a bit more complicated than i though. i need to move everything i want to draw inside the second draw loop.
anyway, does anybody know if it is possible to change the renderer of the canvas/second window ?
i need to draw some textured shapes using vertex and can only do so by using P3D or opengl.

Not sure if this is what you're after. But if you want a small preview of the main window inside the controlP5 window, perhaps using a simple PImage will do the trick. Then you also don't have to write/draw stuff twice.

1. import processing.opengl.*;
2. PImage preview;
3. 
   
4. import controlP5.*;
5. ControlP5 controlP5;
6. ControlWindow controlWindow;
7. 
   
8. void setup() {
9.   size(800,800,OPENGL);
10.   controlP5 = new ControlP5(this);
11.   controlWindow = controlP5.addControlWindow("controlP5window",100,100,width/2,height/2,30);
12.   controlWindow.setUpdateMode(ControlWindow.NORMAL);
13.   controlWindow.addCanvas(new MyCanvas());
14. }
15. 
    
16. void draw() {
17.   background(0);
18.   beginShape();
19.   fill(255,0,0);
20.   vertex(width/5, height/5, -50);
21.   vertex(4*width/5, height/5, -50);
22.   fill(0,0,255);
23.   vertex(4*width/5, 4*height/5, 50);
24.   vertex(mouseX, mouseY, 50);
25.   endShape();
26.   preview = get();
27.   preview.resize(width/4,0);
28. }
29. 
    
30. class MyCanvas extends ControlWindowCanvas {
31.   public void draw(PApplet theApplet) {
32.   theApplet.background(255);
33.   theApplet.image(preview,theApplet.width/4,theApplet.height/4);
34.   }
35. }