Bootstrap 4 can't add gutter between two columns

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I can't add a gutter between 2 columns. I have a row that has to be 1280px wide (using 1366x768 as reference resolution so i give a padding of 43px both sides on the entire page), with two sections that should be 620px wide each with a gutter of 40px in between. But columns are always half of the view (640px) so there's no room for adding a gutter in between (even i thought this was automatic). Default gutter has been already set 20px each side.

<div class="wrapper">
  <div class="row">
    <div class="col-lg-6 row a">
      aaa
    </div>
    <div class="col-lg-6 row b">
      aaa
    </div>
  </div>
</div>
.wrapper {
  padding: 0 43px;
  background: red;
  height: 100vh;
}

.a {
  background: blue;
  margin: 0;
}

.b {
  margin: 0;
  background: green;
}

first of all it is a bad practice to use row class with the col-**-* class.

second thing is that if you are use your custom class with col-**-* class and apply the background-color then it cover the whole area of you column so please put your custom class as child element of your col-**-* class and then apply background-color to your custom class.

show the snippet in full-width:

.wrapper {
  padding: 0 43px;
  background: red;
  height: 100vh;
}

.a {
  background: blue;
  margin: 0;
}

.b {
  margin: 0;
  background: green;
}
<!DOCTYPE html>
<html lang="pt-br">
<head>
  <title>Teste</title>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <meta http-equiv="X-UA-Compatible" content="ie=edge">
  <link rel="stylesheet" type="text/css" href="https://stackpath.bootstrapcdn.com/bootstrap/4.3.1/css/bootstrap.min.css" />
  <link rel="stylesheet" type="text/css" href="css/style.css" />
</head>
<body>
  <div class="wrapper">
    <div class="row">
      <div class="col-lg-6">
        <div class="a">aaa</div>
      </div>
      <div class="col-lg-6">
        <div class="b">bbb</div>
      </div>
    </div>
  </div>
</body>
</html>

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add a container within your wrapper

<div class="wrapper">
 <div class="container"> <!-- or container-fluid -->
  <div class="row">
    <div class="col-lg-6 row a">
      aaa
    </div>
    <div class="col-lg-6 row b">
      aaa
    </div>
  </div>
 </div>
</div>

check also how .container and .container-fluid class works https://getbootstrap.com/docs/4.3/layout/overview/

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as I know to add the gutters you no need to write CSS, this is given by bootstrap its self. just add no gutters in div class like <div class="row no-gutters">

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