Use one counter but give them each a different modulo as to when they should appear. If you want the two to appear together often then use numbers that cross each other often (like 4 and 6). If you don't want them to appear often use prime numbers (like 13 and 17). If they don't appear together as often as you like with either method then add more than 1 to your counter.

yeah i know, something like this (see code below) creates very close rhythms, but this is not phasing ,i think
because relationships between those two are always the same, no?
i am after really small diffrences in loops, something that lags behind other only by 0.005 something etc.
i wish to trigger sound afterwards with each square and achieve phase fx used by Steve Reich and others


float n=0;
void setup(){
size(500,400);
frameRate(10);
}
void draw(){
background(255);
fill(50,200,40);
rectMode(CORNER);
if (n%4==0){
rect(0,0,100,100);
}
if (n%6==0){
rect(400,0,100,100);
}
n+=2;
}

You can simulate modulo while keeping the small difference:
if (n2 > 7)
{
  rect( ... );
  n2 -= 7;
}

but of course! thank you

hello,
another thing...
here 2 squares start in sync, phase out and then at some point came back to starting point and repeats the cycle
how do i calculate how much variations there are in the cycle?
and how to setup things in way that program stops when cycle is done?


float n=0;
float n2=0;
void setup(){
size(500,400);
frameRate(12);
}

void draw(){
background(255);
fill(0);
rectMode(CORNER);
if (n%4==0){ 
rect(0,0,width/2,height/2);
}
if (n2>4){ 
rect(width/2,0,width/2,height/2);

n2-=4;
}
n+=2;
n2+=2.05;


}

Mmm, I improved a bit your sketch to experimentally check when the rectangles seems visually in synch: I click when I see then pulsating simultaneously, displaying info on current state. I also checked when the values are equals, which surprisingly (?) happens when they start to desynchronize again...
Synch seems to happen every 80 frames, and lasts some 36 frames. Not sure how to interpret these results.

1. float n1;
2. float n2;
3. float cycleVal = 4;
4.  
5. void setup() {
6.   size(500, 400);
7.   frameRate(6);
8. }
9.  
10. void draw() {
11.   background(255);
12.   fill(0);
13.   rectMode(CORNER);
14.   if (abs(n1 - n2) < 0.001) println("Equals! " + n1 + " " + n2 + " " + frameCount);
15.   if (n1 > cycleVal) {
16.     rect(0, 0, width/2, height/2);
17.     n1 -= cycleVal;
18.   }
19.   if (n2 > cycleVal) {
20.     rect(width/2, 0, width/2, height/2);
21.     n2 -= cycleVal;
22.   }
23.   
24.   n1 += 2;
25.   n2 += 2.05;
26. }
27.  
28. void mousePressed()
29. {
30.   println(n1 + " " + n2 + " " + frameCount);
31. }