xHTML/CSS: How to make inner div get 100% width minus another div width

I have 2 nested divs inside outer one, which has width:100%. Both nested divs should be in one line and first should get it size from it's contents:

<div id="#outer" style="width:100%; border:1px">
  <div id="#inner1" style="border:1px; display:inline">
    inner div 1. Some text...
  </div>
  <div id="#inner2" style="width:100%????; border:1px; display:inline">
    inner div 2...
  </div>
</div>

Question is how to make #inner2 div to get rest of the horizontal space if width of the #inner1 div is not specified and depends on what it is inside?

P.S. All styles are in separate classes in my case, here I putted CSS into style attributes just for simplification.

I want result to work in IE7+ and FF 3.6

In more details for me it looks like this:

 <style type="text/css">
.captionText
{
 float:left;
} 

.captionLine
{
 height: 1px;
 background-color:black;
 margin: 0px;
 margin-left: 5px;
 margin-top: 5px;
 border: 0px;
 padding: 0px;
 padding-top: 1px;
}
 </style>
<table style="width:300px;">
<caption width="100%">
     <div class="captionText">Some text</div>
     <div class="captionLine"> </div>
</caption>
     <tr>
           <td>something</td>
     </tr>
</table>

Here is the image of what I want: