first import

sites/all/modules/panels/plugins/layouts/twocol/panels-twocol.tpl.php

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+<?php
+/**
+ * @file
+ * Template for a 2 column panel layout.
+ *
+ * This template provides a two column panel display layout, with
+ * each column roughly equal in width.
+ *
+ * Variables:
+ * - $id: An optional CSS id to use for the layout.
+ * - $content: An array of content, each item in the array is keyed to one
+ *   panel of the layout. This layout supports the following sections:
+ *   - $content['left']: Content in the left column.
+ *   - $content['right']: Content in the right column.
+ */
+?>
+<div class="panel-display panel-2col clearfix" <?php if (!empty($css_id)) { print "id=\"$css_id\""; } ?>>
+  <div class="panel-panel panel-col-first">
+    <div class="inside"><?php print $content['left']; ?></div>
+  </div>
+
+  <div class="panel-panel panel-col-last">
+    <div class="inside"><?php print $content['right']; ?></div>
+  </div>
+</div>

sites/all/modules/panels/plugins/layouts/twocol/twocol.css

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+
+.panel-2col { 
+/*  overflow: hidden;  */
+}
+
+.panel-2col .panel-col-first { 
+  float: left; 
+  width: 50%; 
+}
+* html .panel-2col .panel-col-first {
+  width: 49.9%;
+}
+
+.panel-2col .panel-col-first .inside { 
+  margin: 0 .5em 1em 0;
+}
+
+.panel-2col .panel-col-last { 
+  float: left; 
+  width: 50%; 
+}
+* html .panel-2col .panel-col-last {
+  width: 49.9%;
+}
+
+.panel-2col .panel-col-last .inside { 
+  margin: 0 0 1em .5em;
+}
+
+#panels-edit-display .panel-pane,
+#panels-edit-display .helperclass {
+  margin: .5em;
+}
+
+.panel-2col .panel-separator {
+  margin: 0 0 1em 0;
+}

sites/all/modules/panels/plugins/layouts/twocol/twocol.inc

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+<?php
+
+// Plugin definition
+$plugin = array(
+  'title' => t('Two column'),
+  'category' => t('Columns: 2'),
+  'icon' => 'twocol.png',
+  'theme' => 'panels_twocol',
+  'css' => 'twocol.css',
+  'regions' => array(
+    'left' => t('Left side'),
+    'right' => t('Right side')
+  ),
+);