You can try this :

<form
    class="flex-list"
    [formGroup]="calculation_Input"
    (input)="input(&event)"
  
  >
    <div class="range-slider">
      <input
        id="slider3"
        name="in1"
        class="range-slider__range"
        type="range"
        value.equity="equity"
        min="0"
        max="500"
      />
      <span class="range-slider__value"> 0</span>
    </div>

    <div class="range-slider">
      <input
        id="slider3"
        class="range-slider__range"
        type="range"
        name="in2"
        value.equity="equity"
        min="0"
        max="500"
      />
      <span class="range-slider__value"> 0</span>
    </div>
</form>

event call :

input(e: any): void {
  if(e.target.name == "in1")
    console.log("for fist input value is : " + event.target.value)
  else console.log("for second input value is : " + event.target.value)
}

Try using [formControl] attribute on each of the inputs and then you can use valueChanges and subscribe.

<form
    class="flex-list"
    [formGroup]="calculation_Input"
  >
    <div class="range-slider">
      <input
        id="inputRangeStart"
        [formControl]="inputRangeStart"
        class="range-slider__range"
        type="range"
        value.equity="equity"
        min="0"
        max="500"
      />
      <span class="range-slider__value"> 0</span>
    </div>

    <div class="range-slider">
      <input
        id="inputRangeEnd"
        class="range-slider__range"
        type="range"
        [formControl]="inputRangeEnd"
        value.equity="equity"
        min="0"
        max="500"
      />
      <span class="range-slider__value"> 0</span>
    </div>
</form>

Event Call: // some.component.ts

inputRangeStart = new FormControl();
inputRangeEnd = new FormControl();

ngOnInit(): void {
 this.inputRangeStart.valueChanges.subscribe(value => this.onInput(value))
 this.inputRangeEnd.valueChanges.subscribe(value => this.onInput(value))
}
onInput(value: string){
    // do something
}

I have two input fields and they refer to the same event.target.value object. I want to have them separatly.

  <form
    class="flex-list"
    [formGroup]="calculation_Input"
    (input)="input($event)"
  >
    <div class="range-slider">
      <input
        id="slider3"
        class="range-slider__range"
        type="range"
        value.equity="equity"
        min="0"
        max="500"
      />
      <span class="range-slider__value"> 0</span>
    </div>

    <div class="range-slider">
      <input
        id="slider3"
        class="range-slider__range"
        type="range"
        value.equity="equity"
        min="0"
        max="500"
      />
      <span class="range-slider__value"> 0</span>
    </div>
</form>

The event.target.value refers now to both inputs and I have no clue how I can change it.

  input(e: any): void {
console.log(event.target.value)
  }