LCD ProgressBar (visible numbers instead of squares)

LiquidCrystal_I2C lcd(0x27,2,1,0,4,5,6,7);
LiquidCrystal_I2C lcd_1(0x26,2,1,0,4,5,6,7);

Do you count that way? When you feel the need to number variables in a set, number ALL of them.

#define lenght 16.0
Sppeling deos os matr.

In this case, though, use better names. One device displays a progress bar, so name the instance progressBar.

void Sensor_0(){

What a dumb name for a function. There is NOT a clue as to what kind of sensor or what this function does.

    for (int i=1;i<a;i++) {
    lcd.write(4);
    b=i;
    }

At the end of this loop, won't b have a fixed value (at least relative to a)?

    a=a-b;

And what will a now be?

Convoluted logic to achieve a simple result is NOT a good thing.

  peace=a*5;
  switch (peace) {
  case 0:
    break;
  case 1:
    lcd.print(0);
    break;
  case 2:
    lcd.print(1);
    break;
  case 3:
    lcd.print(2);
    break;
  case 4:
    lcd.print(3);
    break;
  }

Well, now we know where the 0, 1, 2, and 3 are coming from. Perhaps you meant to write() the special characters 0, 1, 2, and 3.