Edit in JSFiddle

<h2>Split</h2>

  
<h3>Simple split</h3>

<ul>
  <li>Limited to 2 elements</li>
</ul>

<div class="split --responsive">
  <div>a</div>
  <div>b</div>
</div>


<h3>Split using table layout</h3>

<ul>
  <li>Unlimited elements</li>
  <li>You have controll over vertical alignment</li>
</ul>

<div class="split --table --responsive">
  <div>a</div>
  <div>b</div>
  <div>c</div>
  <div>d<br>e<br>f<br></div>
</div>



<h2>Join</h2>

<div class="join --responsive">
  <span>Home</span>
  <span>About</span>
  <span>Contact</span>
</div>

<h2>Sample usage</h2>

<div class="split --table --responsive">
  <div>
    <a class="logo" href="#">Site name</a>
  </div>
  <ul class="nav join --small">
      <li><a href="#">Home</a></li>
      <li><a href="#">Projects</a></li>
      <li><a href="#">About</a></li>
      <li><a href="#">Contact</a></li>
  </ul>
</div>

$spacing-unit: 30px;
$breakpoint:   480px;

.clearfix,
%clearfix {

  &:after {
    content: "";
    display: table;
    clear: both;
  }
} 

// Split object

.split {
  @extend %clearfix;

  > * {
    float: left;
  }

  > * + * {
    float: right;
  }


  &.\--table {
    display: table;
    width: 100%;
    table-layout:fixed;
    text-align: center;

    &:after {
      content: none;
    }

    > * {
      float: none;
      display: table-cell;
      vertical-align: middle;
    }

    > :first-child {
      text-align: left;
    }

    > :last-child {
      text-align: right;
    }
  }


  &.\--responsive {
    @media screen and (max-width: $breakpoint) {
      display: block;
      text-align: center;

      > * {
        float: none;
        display: inline-block;
      }

      > * + * {
        display: block;
        margin-top: $spacing-unit/2;
      }

      > :first-child,
      > :last-child {
        text-align: inherit;
      }
    }
  }
}

// Join object

.join {
  list-style-type: none;

  > * {
    display: inline-block;
    vertical-align: top;
  }

  > * + * {
    margin-left: $spacing-unit;
  }

  &.\--middle > * { vertical-align: middle; }
  &.\--bottom > * { vertical-align: bottom; }

  &.\--tiny  > * + * { margin-left: $spacing-unit/4; }
  &.\--small > * + * { margin-left: $spacing-unit/2; }
  &.\--large > * + * { margin-left: $spacing-unit*2; }

  &.\--force {
    @extend %clearfix;

    > * {
      float: left;
    }
  }

  &.\--responsive {
    @media screen and (max-width: $breakpoint) {
      text-align: center;

      > * {
        float: none;
        display: inline-block;
      }

      > * + * {
        display: block;
        margin-left: 0;
        margin-top: $spacing-unit/2;
      }
    }
  }
}


// ****************
// Styling
// ****************

body {
    font-family: Helvetica, Arial, sans-serif;
  color: #666;
  max-width: 720px;
  margin: 30px auto;
  border: 1px solid #e5e5e5;
  padding: 1em;
  
}

h2 {
  margin-top: 0;
  margin-bottom: 30px;
  color: #222;
  border-left: 5px solid #87D37C;
  padding-left: 15px;
  
  & ~ h2 {
    margin-top: 30px;
  }
}

h3 {
  color: #444;
  margin: 0 0 15px 0;
  border-left: 5px solid #ccc;
  padding-left: 15px;
  
  & ~ h3 {
    margin-top: 30px;
  }
}
  
p, ul {
  color: #999;
  line-height: 18px;
  font-size: 12px;
  margin-bottom: 30px;
  font-weight: 300;
}

ul {
  list-style-position: inside;
  padding-left: 20px;
}

p + ul {
  margin-top: -20px;
}

.logo {
  text-decoration: none;
  color: #5C97BF;
  font-size: 24px;
  font-weight: bold;
}

.nav {
  margin: 0;
  list-style-type: none;
  
  a {
    text-decoration: none;
    color: #666;
    
    &:hover {
      text-decoration: underline;
    }
  }
}