I tried to setup to packs for sending data to the gui: one with a '0' and one with '1' at the beginning. But I received only buffer[0]. Any ideas? Not a big problem, because I can pack all data together of course.

It should work.

For instance, take the bela-to-gui example and apply the following changes:

to the.cpp file:

@@ -63,7 +63,7 @@ void render(BelaContext *context, void *userData)
                {
                    gNumber++;
                        gui.sendBuffer(0, gNumber);
-
+                       gui.sendBuffer(1, gNumber + 1);
                        //and we reset the counter
                        count = 0;
                }

and to the js side:

@@ -28,7 +28,7 @@ function setup() {
 function draw() {

        //Read buffer with index 0 coming from render.cpp.
-       let counter = Bela.data.buffers[0];
+       let counter = Bela.data.buffers[0] + '+' + Bela.data.buffers[1];

        //We change the background color
        //When starting the program, if Bela hasn't sent any value yet, counter could take the value NaN (not a number),
@@ -44,7 +44,7 @@ function draw() {
        //Format and display text
        fill(100, 0, 255);
        //Adjust the size of the text to the window width
-       textSize(windowWidth / 4);
+       textSize(windowWidth / 10); // ensure we see the whole thing on screen
        //Display text
        text(counter, 0.4 * (windowWidth), windowHeight / 2);

I agree, in C++ it should work, but I'm wondering about PD? I packed them into 2 different packs and send those packs to separate . Maybe i should send them ton the same ?

oh you are right it was broken. It's now fixed on dev, with the updated PureData/gui-send example showcasing it.

In your sketch.js this line should be
let numCircles =pdBuf0.length+pdBuf1.length;
instead of:
let numCircles = min(pdBuf0.length, pdBuf1.length);
Otherwise you'll see only 2 circles.
Anyway, thanks for the update - it works now.

It's meant to be min() so that only two circles are seen. The two elements of pdBuf0 affect the stroke width while the two elements from pdBuf1 affect the brightness of the stroke. If you make it + the for loop will throw a runtime error.

If you meant to see only 2 circles, than it's OK. But I don't see any runtime error.

right, no runtime errors but you get circles 1, 2 and 3 that are all linked together: trying to access pdBuf0[i] or pdBuf1[i] when i >= 2 will return undefined and the calls to stroke() and strokeWeight will be ignored by p5, so these two circels will be the same as the last valid one (i = 1)