Well, first thing we should do is move to FastLED ... I did this quickly based on your code.
So ... this is most likely not the final answer to your question, but it will get us there eventually 😊 

Note: I wasn't able to test, since I do not have the hardware laying around, but it compiles without errors.

Cleaned up formatting and migrated to FastLED:

#define FASTLED_INTERNAL       // just used to mute the Pragma messages when compiling
#include "FastLED.h"
#define PIN 2
#define NUM_LEDS 425
CRGB leds[NUM_LEDS];

int inputPin = 7;              // choose the input pin (for PIR sensor1)
int inputPin2 = 4;             // choose the input pin (for PIR sensor2)
int pirState = LOW;            // we start, assuming no motion detected
int pirState2 = LOW;           // we start, assuming no motion detected2
int val = 0;                   // variable for reading the pin status
 
void setup() {
  FastLED.addLeds<WS2811, PIN, GRB>(leds, NUM_LEDS).setCorrection( TypicalLEDStrip );
  
  FastLED.clear();  // clear all pixel data
  FastLED.show();
  
  pinMode(inputPin, INPUT);     // declare sensor as input
  pinMode(inputPin2, INPUT);     // declare sensor as input
 
  Serial.begin(9600);
}
 
void loop(){
  if (digitalRead(inputPin) == HIGH) {   // alttan çıkan kontolü
    Serial.println("alttan cikan!");
    
    meteorRain(0xB7,0x00,0xFE,5, 64, true, 0);
    colorWipe(0xFF,0xB1,0x6E, 10);
    delay(1000);
    colorWipe(0x00,0x00,0x00, 10);// wipe led off
    
    if (pirState == LOW) {  // we have just turned on
      Serial.println("alttan cikan");  // We only want to print on the output change, not state
      pirState = HIGH;
    } 
  }
 
  if (digitalRead(inputPin2) == HIGH) {
    Serial.println("üstten inen!");
    
    meteorRaindown(0xFF,0x00,0x00,5, 64, true, 0);
    colorWipedown(0xFF,0xB1,0x6E, 10);
    delay(1000);
    colorWipedown(0x00,0x00,0x00, 10);// wipe led off
    
    if (pirState == LOW) {
      // we have just turned on
      Serial.println("üstten inen");
      // We only want to print on the output change, not state
      pirState = HIGH;
    } 
  }
}

void meteorRain(byte red, byte green, byte blue, byte meteorSize, byte meteorTrailDecay, boolean meteorRandomDecay, int SpeedDelay) {  
  FastLED.clear();
 
  for(int i = 0; i < NUM_LEDS+NUM_LEDS; i++) {   
    // fade brightness all LEDs one step
    for(int j=0; j<NUM_LEDS; j++) {
      if( (!meteorRandomDecay) || (random(10)>5) ) {
        leds[j].fadeToBlackBy( meteorTrailDecay );        
      }
    }
   
    // draw meteor
    for(int j = 0; j < meteorSize; j++) {
      if( ( i-j <NUM_LEDS) && (i-j>=0) ) {
        leds[i-j] = CRGB(red, green, blue);
      }
    }
   
    FastLED.show();
    delay(SpeedDelay);
  }
}

void meteorRaindown(byte red, byte green, byte blue, byte meteorSize, byte meteorTrailDecay, boolean meteorRandomDecay, int SpeedDelay) {  
  FastLED.clear();
 
  for(int i = 0; i < NUM_LEDS+NUM_LEDS; i++) {
   
   
    // fade brightness all LEDs one step
    for(int j=0; j<NUM_LEDS; j++) {
      if( (!meteorRandomDecay) || (random(10)>5) ) {
        leds[j].fadeToBlackBy( meteorTrailDecay );        
      }
    }
   
    // draw meteor
    for(int j = 0; j < meteorSize; j++) {
      if( ( i-j <NUM_LEDS) && (i-j>=0) ) {
        leds[NUM_LEDS-i-j]=CRGB(red, green, blue); // <-- change here
      } 
    }
   
    FastLED.show();
    delay(SpeedDelay);
  }
}

void colorWipe(byte red, byte green, byte blue, int SpeedDelay) {
  for(uint16_t i=0; i<NUM_LEDS; i++) {
      leds[i] = CRGB(red, green, blue);
      FastLED.show();
      delay(SpeedDelay);
  }
}
void colorWipedown(byte red, byte green, byte blue, int SpeedDelay) {
  for(uint16_t i=NUM_LEDS; i < -1; i--) {
      leds[i] = CRGB(red, green, blue);
      FastLED.show();
      delay(SpeedDelay);
  }
}

Next thing I noticed is that your void loop() is a little bit mixed up it seems.

You defined 2 variables for the PIR states, but for example "pirState" has a value assigned and you set it to HIGH at some point, but never back to LOW or anything like that. I suspect this was intended to store the PIR state of the first PIR. Likewise the variable pirState2 was defined but never used. It is a little confusing what is going on there.

Maybe you're looking for something like this, which is just a wild guess, since I wasn't able to test this.

void loop(){
  pirState = digitalRead(inputPin);
  
  if (pirState == HIGH) {   // alttan çıkan kontolü
    Serial.println("alttan cikan!");
    
    meteorRain(0xB7,0x00,0xFE,5, 64, true, 0);
    colorWipe(0xFF,0xB1,0x6E, 10);
    delay(1000);
    colorWipe(0x00,0x00,0x00, 10);// wipe led off
  }

  pirState2 = digitalRead(inputPin2);
  
  if (pirState2 == HIGH) {
    Serial.println("üstten inen!");
    
    meteorRaindown(0xFF,0x00,0x00,5, 64, true, 0);
    colorWipedown(0xFF,0xB1,0x6E, 10);
    delay(1000);
    colorWipedown(0x00,0x00,0x00, 10);// wipe led off
  }
}

Note: I do realize that during testing and tinkering, code can become a little messy. 
When things do not quite work as expected, it is always a good idea to clean up and format code, so it becomes more readable. 😊 

@hans Thank you for your help and time. 

I get the codes you shared on the effects post and try to change them such as meteor rain up and down and PIR sensors etc. Did not write them myself since my knowledge about coding is limited, just copied them from another projects that is close to what I want. 

If you see something that is odd, it's probably my mistake not intended, but the code I posted seems to be working without any issues (or I haven't noticed any mulfunction yet) just slow on long strips.

About the PIR states, I have one at the bottom of the stairs and another at the top. (I wrote "alttan cikan and üstten inen to find out which one is which meaning walking up or down) So their codes teorically should be the same, just inverse. I haven't noticed the lack of usage for PIR state2   🙄 

I'll try the codes that you edited for me in a short time and see if it works as planned.

Thanks again appriciate it.

Cool - let me know if you run into a problem or not 😊