Use this technique to 'float' two tables next to each other using the html align attribute. 
If you want something to be 100% width though, the only way to guarantee it is to go with a single column with width="100%". As Gmail doesn't accept media queries, there is no way to do both float/align and full width. 
Note - In my example I've used 320px as that is the width of most Android phones, so when the tables stack, it will appear full width.
Edit: here is a full email example:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">
<html><head><meta http-equiv="Content-Type" content="text/html; charset=utf-8"><title></title>
  <style type="text/css">           
    /* Client-specific Styles - last updated 18 July 2013 */
    #outlook a {padding:0;}
    body{width:100% !important; -webkit-text-size-adjust:100%; -ms-text-size-adjust:100%; margin:0; padding:0;} /* force default font sizes */
    .ExternalClass {width:100%;} .ExternalClass, .ExternalClass p, .ExternalClass span, .ExternalClass font, .ExternalClass td, .ExternalClass div {line-height: 100%;} /* Hotmail */
    a[href^="tel"], a[href^="sms"] { text-decoration: default; color: #000001 !important; pointer-events: auto; cursor: default;}
    table td {border-collapse: collapse;}
    .msoFix {
      mso-table-lspace:-1pt;
      mso-table-rspace:-1pt;
   }
  </style>
</head>
<body style="margin: 0px; padding: 0px; background-color: #FFFFFF;" bgcolor="#FFFFFF">
<table width="100%" border="0" align="center" cellpadding="0" cellspacing="0">
  <tr>
    <td align="center">
      <div style="max-width:640px !important;">
        <table class="msoFix" width="320" cellpadding="0" cellspacing="0" align="left" bgcolor="#CCCCCC">
          <tr>
            <td width="15" bgcolor="454545"> </td>
            <td width="290" bgcolor="454545" style="padding: 0px;"> <br>table 1<br>...<br> 
            </td>
            <td width="15" bgcolor="454545"> </td>
          </tr>
        </table>
        <table class="msoFix" width="320" cellpadding="0" cellspacing="0" align="left" bgcolor="#EEEEEE">
          <tr>
            <td width="15" bgcolor="959595"> </td>
            <td width="290" bgcolor="959595" style="padding: 0px;"> <br>table 2<br>...<br> 
            </td>
            <td width="15" bgcolor="959595"> </td>
          </tr>
        </table>
      </div>
    </td>
  </tr>
</table>
</body></html>