Using two flow sensors connected to one Arduino.

Using Arduino Sensors
3

Something like this?

#include <Wire.h>
#include <LiquidCrystal_I2C.h>
LiquidCrystal_I2C lcd(0x27,16,2);

byte sensorInterruptA  = 0;  
byte sensorInterruptB  = 1;
byte sensorPinA        = 2;
byte sensorPinB        = 3;

float calibrationFactor = 50; 

volatile byte pulseCount;

float flowRate;
unsigned int turFuel;
unsigned int returFuel;

unsigned long oldTime;

void setup()
{
  lcd.begin(16, 2);
  lcd.setCursor(0, 0);
  lcd.print("                ");
  lcd.setCursor(0, 1);
  lcd.print("                ");
  
  Serial.begin(38400); 
  
  pulseCount    = 0;
  flowRate      = 0.0;
  turFuel       = 0;
  returFuel     = 0;
  oldTime       = 0;
  
  attachInterrupt(sensorInterruptA, pulseCounter, FALLING);
  attachInterrupt(sensorInterruptB, pulseCounter, FALLING);
}

//hovedprogrammet i loop. 

void loop()
{
  if((millis() - oldTime > 1000)
  {
    detachInterrupt(sensorInterrupt);
    
    flowRate = ((1000.0 / (millis() - oldtime)) * pulseCounat) / calibrationFactor;
    
    oldTime =millis();
    
    flowrate = (turFuel - returFuel) / 8500;
    
    unsigned int frac;
    
    Serial.print(int(flowRate));
    Serial.print(".");            
    frac = (flowRate - int(flowRate)) * 10;
    Serial.print(frac, DEC) ;
    
    Serial.print(" ");             
    Serial.print(flowRate);
    
     lcd.setCursor(0, 0);
    lcd.print("                ");
    lcd.setCursor(0, 0);
    lcd.print("Flow: ");
    if(int(flowRate) < 10)
    {
      lcd.print(" ");
    }
    lcd.print((int)flowRate);   
    lcd.print('.');             
    lcd.print(frac, DEC) ;      
    lcd.print(" L");
    lcd.print("/min");
    
    pulseCount = 0;
    
    attachInterrupt(sensorInterrupt, pulseCounterA, FALLING);
    attachInterrupt(sensorInterrupt, pulseCounterB, FALLING);
  }
}

void pulseCounter()
{
  pulseCount++;
}