thead with one td (full width) and tbody with 2 columns (width not depending on thead)

<table>
  <thead>
     <tr><th colspan="2">longer Heading with a width of 100%</th></tr>
  </thead>
  <tbody>
     <tr><td>cell 1</td><td>cell 2 </td></tr>
      <tr><td>cell 3</td><td>cell 4 </td></tr>
 </tbody>

use of colspan will do the trick for you

This?

<th colspan="2">...</th>

It's easier than that: You just need a colspan on your th element.
Colspan defines, how many cells the element extends:

<table>
    <thead>
        <tr><th colspan="2">longer Heading with a width of 100%</th></tr>
    </thead>
    <tbody>
        <tr><td>cell 1</td><td>cell 2</tr>
        <tr><td>cell 3</td><td>cell 4</tr>
    </tbody>
</table>

See my Fiddle