<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink">
  <path id="thePath" d="M50 150 L 450 150" />
  <text x="50" y="100">This text should be ABOVE the other line.</text>
  <text>
    <textPath id="theTP" xlink:href="#thePath">This text should be BELOW the other line.</textPath>
  </text>
</svg>