Kleiner Workaround:
<html>
<head>
<style>
td {
text-align: center;
width: 20%;
}
</style>
</head>
<body>
<table border="1" width="100%">
<tr>
<td colspan="5" style="width: 50%;">breit</td><td colspan="5" style="width: 50%;">breit</td>
</tr>
<tr>
<td colspan="2">schmal</td><td colspan="2">schmal</td><td colspan="2">schmal</td><td colspan="2">schmal</td><td colspan="2">schmal</td>
</tr>
<tr>
<td colspan="2">schmal</td><td colspan="2">schmal</td><td colspan="2">schmal</td><td colspan="2">schmal</td><td colspan="2">schmal</td>
</tr>
</table>
</body>
</html>