JSFiddle - React, Tailwind, and code Playground

HTML

<input type="text" name="A[1]">
<input type="text" name="A[2]">
<input type="text" name="B[1]">
<input type="text" name="B[2]">

JavaScript

$('input[type=text]').on('keyup', function() {
    var cur = this.name;
    if(cur.indexOf('A') == 0){
       var target = 'B' + cur.replace('A','');
    } else target = 'A' + cur.replace('B','');
    var targetVal = $('input[name="'+ target +'"]').val();
    var sum = parseInt(this.value) + parseInt(targetVal ? targetVal: 0);
    console.log(sum);
});