Juhhu, es Funktioniert
Ich habe die ganze Flankentriggerung weggelassen und die Interrupts nun heiss auf "any logical change" gemacht...

Vielen Dank nochmals für dienen Hilfe!

Hier der funktionierende Code:
Code:
#include <inttypes.h> 
#include <avr/interrupt.h>

#define F_CPU 8000000      /* 8Mhz */
#include <util/delay.h>    /* definiert _delay_ms() ab avr-libc Version 1.2.0 */
#include "twimaster.c"

#include "LCD.h"

volatile long t2_y,t2_x;
volatile long t1_y,t1_x;

volatile int x1,x0; // Temp Variablen für ISR's

int count_y, count_x;
char ziel[5];

ISR(SIG_INTERRUPT1){ 
GIFR = 0;
  if(x1==1){
    x1=0;	
    t1_y = count_y;
  }
  
  else{
    x1=1;
  	t2_y = count_y;
    count_y = 0;
	}
}

ISR(SIG_INTERRUPT0){ 
GIFR = 0;
  if(x0==1){		 
    x0=0;	
    t1_x = count_x;
  }
  
  else{
    x0=1;
    t2_x = count_x;
    count_x = 0;
	}
}

ISR (SIG_OUTPUT_COMPARE1A){
count_y++;
count_x++;
}  

void Timer_Inits(void){

    TCCR1A = 0;
    TCCR1B = (1 << WGM12) | (1 << CS10); 
  
  	OCR1A = (uint16_t) ((uint32_t) F_CPU / 50000); //100000

    TIMSK |= (1 << OCIE1A); 	// Interrupt wenn Timer Vergleichswert erreicht

	//Jede logische änderung von INT0 oder INT1 löst Interrupt aus
	MCUCR = (0<<ISC11 | 1<<ISC10 | 0<<ISC01 | 1<<ISC00);

 	GIMSK = (1<<INT0 | 1<<INT1);		//Int0 u. 1 aktiv

} 

int main (void){
	DDRC = 0x00;    //alles als Eingan def.
	PORTC =0x00;	//PullUp's aus

	DDRD  =0x00;	//alles Eingang
	PORTD =0x00;	//PullUp's aus
	
	Timer_Inits();

	i2c_init(); 
	_delay_ms(10000);
	lcd_init();
	_delay_ms(10000);

	sei();

for(;;){

_delay_ms(90000000);
clear();

//A(g) = ((T1/T2) / 0.5)/12.5%

int resultx,resulty;

   uint8_t sreg = SREG;
   cli();

   long t1x = t1_x;
   long t2x = t2_x;
   long t1y = t1_y;
   long t2y = t2_y;
   SREG = sreg;

   if(t2x!=0) resultx = (((((float)t1x/(float)t2x)*1000)-481)*1000)/125; 
   else resultx=0;

   if(t2y!=0) resulty = (((((float)t1y/(float)t2y)*1000)-481)*1000)/125; 
   else resulty=0;
   sei();

	sprintf(ziel,"%d",resultx);
	stringout ("ay=");
	stringout (ziel);

	sprintf(ziel,"%d",resulty);
	stringout (" ax=");
	stringout (ziel);
   }
}