Fading led's in sequence not together.

I think that confused me more I tried something like this but i am pretty sure its wrong.

#include <math.h>

unsigned long previousMillis = 0; // last time update
long interval = 2000; // interval at which to do something (milliseconds)

int led1 = 11;
int led2 = 10;

void setup()
{
  pinMode(led1, OUTPUT);
  pinMode(led2, OUTPUT);
  pinMode(9, OUTPUT);
  pinMode(6, OUTPUT);
}

void loop(){
  unsigned long currentMillis = millis();
  if (currentMillis - previousMillis >= interval){
    previousMillis = previousMillis + interval;
    led1 = led1+1;
    if (led1== 3){led1 = 3;}  // reset pointer, say max = 3 for this example
   
    float val = (exp(sin(millis()/2000.0*PI)) - 0.36787944)*108.0;
    
switch(led1){
case 0:
analogWrite(led1, val);

break;
case 1:
analogWrite(led2, val);

} // end switch
} // end time check
} // end loop