你需要改变你的html的结构，也改变宽度：40%到宽度：40vw，你应该都设置好了

@import url('https://fonts.googleapis.com/css?family=Roboto+Condensed:700|Roboto:100,300,700');
html,
body {
  margin: 0;
  padding: 0;
  font-family: Roboto;
  font-size: 10px;
  line-height: 3.5;
  font-weight: 300;
}

h1,
h2,
h3,
h4,
h5 {
  font-family: Roboto Condensed;
}

h1 {
  font-size: 9em;
  text-align: center;
  text-transform: uppercase;
}

h2 {
  font-size: 5em;
  text-align: center;
  text-transform: uppercase;
  line-height: 4em;
  color: white;
  text-shadow: #020819 8px -20px 9px
}

h3 {
  font-size: 4.2em;
  text-align: center;
  line-height: 1em;
  margin-top: 0;
}

h4 {
  font-size: 1.5em;
  letter-spacing: 0.4;
  line-height: 1em;
}

h5 {
  font-size: 1.2em;
  line-height: 1em;
  margin-top: 0;
  margin-bottom: 0;
}

.service-box {
  font-size: 1.7em;
  text-align: center;
  border: 2px solid #eee;
  border-radius: 20px;
  
  box-shadow: 2px 2px 10px 0 #eee;
  margin: 50px 5% 0 5%;
  padding: 20px;
  width: 40vw;
}

.service-box img {
  width: 150px;
}

.structure {
  display: flex;
  justify-content: space-between;
  flex-direction: row;
  flex-wrap: wrap;
}

<section class="structure ">

  <div>
    <article class="service-box">
      <img src="https://s3-eu-west-1.amazonaws.com/ih-materials/uploads/ironskydive-training.png">
      <h3>Training</h3>

      <p>We teach all the necessary things to jump from the plane without any kind of problem.</p>

    </article>
  </div>
  <div>
    <article class="service-box">
      <img src="https://s3-eu-west-1.amazonaws.com/ih-materials/uploads/ironskydive-get-ready.png">
      <h3>Get Ready</h3>

      <p>You are already prepared, you just need the suit and a parachute. All sizes available.</p>
    </article>
  </div>
</section>
<section class="structure ">
  <div>
    <article class="service-box">
      <img src="https://s3-eu-west-1.amazonaws.com/ih-materials/uploads/ironskydive-fly.png">
      <h3>Fly</h3>

      <p>You are ready, and the plane is waiting for us in the hangar. Let's fly</p>
    </article>
  </div>
  <div>
    <article class="service-box">
      <img src="https://s3-eu-west-1.amazonaws.com/ih-materials/uploads/ironskydive-jump.png">
      <h3>Jump!</h3>

      <p>You have done the most complicated. just one step left... jump!!</p>
    </article>

  </div>

</section>

你想要一个2x2的网格，因此你应该使用网格而不是Flexbox。

#structure  {
  display: grid;
  grid-template-columns: 1fr 1fr;
}

@import url('https://fonts.googleapis.com/css?family=Roboto+Condensed:700|Roboto:100,300,700');

html,body{
  margin:0;
  padding:0;
  font-family: Roboto;
  font-size: 10px;
  line-height:3.5;
  font-weight:300;
}

h1,h2,h3,h4,h5{
  font-family:Roboto Condensed;
}
h1{
  font-size:9em;
  text-align:center;
  text-transform:uppercase;
}
h2{
  font-size:5em;
  text-align:center;
  text-transform:uppercase;
  line-height:4em;
  color:white;
  text-shadow: #020819 8px -20px 9px
}
h3{
  font-size:4.2em;
  text-align:center;
  line-height:1em;
  margin-top:0;
  
}
h4{
  font-size:1.5em;
  letter-spacing:0.4;
  line-height:1em;
  
  
}
h5{
  font-size:1.2em;
  line-height:1em;
  margin-top:0;
  margin-bottom:0;
}

.service-box{
  font-size:1.7em;
  text-align:center;
  border:2px solid #eee;
  border-radius:20px;
  boxing-size:border-box;
  box-shadow: 2px 2px 10px 0 #eee;
  margin:50px 5% 0 5%;
  padding:20px;
  /* width:40% */
}
.service-box img{
  width:150px;
}

#structure  {
  display: grid;
  grid-template-columns: 1fr 1fr;
}

<section id="structure">

  <article class="service-box">
    <img src="https://s3-eu-west-1.amazonaws.com/ih-materials/uploads/ironskydive-training.png">
    <h3>Training</h3>
    <p>We teach all the necessary things to jump from the plane without any kind of problem.</p>
  </article>

  <article class="service-box">
    <img src="https://s3-eu-west-1.amazonaws.com/ih-materials/uploads/ironskydive-get-ready.png">
    <h3>Get Ready</h3>
    <p>You are already prepared, you just need the suit and a parachute. All sizes available.</p>
  </article>

  <article class="service-box">
    <img src="https://s3-eu-west-1.amazonaws.com/ih-materials/uploads/ironskydive-fly.png">
    <h3>Fly</h3>
    <p>You are ready, and the plane is waiting for us in the hangar. Let's fly</p>
  </article>

  <article class="service-box">
    <img src="https://s3-eu-west-1.amazonaws.com/ih-materials/uploads/ironskydive-jump.png">
    <h3>Jump!</h3>
    <p>You have done the most complicated. just one step left... jump!!</p>
  </article>

</section>