Just understood that this probably was a stupid question since you actually render to a specific PGraphics object. But how can I render to two PGraphics at the same time with one of them at a higher resolution?

At the same time I dunno. But we can get() a PImage copy from the canvas, and then resize() it.

size(600, 400);

PImage higher = get();
higher.resize(800, 600);

Basically, you want to reproduce what the PDF library (and DXF, and SVG, etc.) are doing.
Well, you can create such library, too...

Or use some trick, like using the g variable to substitute its existing PGraphics with the one you want.

With a bit more time, I can show what I mean:

1. OffScreenRender screenRender;
2.  
3. void setup()
4. {
5.   size(1024, 768);
6.   screenRender = new OffScreenRender(this, width, height);
7.   noLoop();
8. }
9.  
10. void draw()
11. {
12.   background(0, 0, 0);
13.  
14.   //Render the graphics to an offScreen buffer
15.   screenRender.startRender();
16.  
17.   fill(#FF0000);
18.   stroke(#00FF00);
19.   rect(600, 50, 230, 150);
20.  
21.   //Stop the render and save image
22.   screenRender.stopRender();
23. }
24.  
25. /**
26.  * Renders the processingsketch to a .jpg
27.  */
28. class OffScreenRender
29. {
30.   PApplet parent; // The parent PApplet
31.   PGraphics render;
32.   PGraphics original;
33.   int width;
34.   int height;
35.  
36.   OffScreenRender(PApplet p, int width_, int height_)
37.   {
38.     //Initialize variables
39.     parent = p;
40.     width = width_;
41.     height = height_;
42.  
43.     render = parent.createGraphics(width, height);
44.     
45.     original = parent.g;
46.   }
47.  
48.   //Render the image
49.   void startRender()
50.   {
51.     parent.g = render;
52.     render.beginDraw();
53.   }
54.  
55.   //Render the image
56.   void stopRender()
57.   {
58.     render.endDraw();
59.     render.save("H:/Temp/hej.jpg");
60.     parent.g = original;
61.   }
62. }

Old trick, still effective...

Thank you for your reply!