.flex {
  max-width: 1300px;
  display: flex;
  flex-wrap: wrap;
  justify-content: space-between;
}
.spalte1 {
  flex-basis: 100%;
  flex-grow: 1;
}
.spalte2 {
  flex-basis: calc(45% - 10px);
  margin-left: 20px;
  margin-right: 20px;
  min-width: 400px;
  flex-grow: 1;
}
@media screen and (max-width: 900px) {
  .flex {
    display: block;
    margin: 0;
    padding: 0px;
  }

  .spalte1,
  .spalte2 {
    width: 100%;
    margin: 30px auto;
    min-width: auto;
  }
  h1.spalte1 {
    padding-left: 1em;
  }
}