In the Insert record behavior, you can only bind a database column to one form element at a time, but you can overcome this by using copy and past.
First bind the search_xtra column to the search_xtra select list, this will add code similar to:
<?php echo(isset($_POST['search_xtra'])?$_POST['search_xtra']:"" ?>
copy the code that is created, then bind the search_xtra column to the search_xtra2 select list. This will create code similar to:
<?php echo(isset($_POST['search_xtra2'])?$_POST['search_xtra2']:"" ?>
place you cursor at the beginning of that code, then paste the code you copied previosly, so the code looks like:
<?php echo(isset($_POST['search_xtra'])?$_POST['search_xtra']:"" ?><?php echo(isset($_POST['search_xtra2'])?$_POST['search_xtra2']:"" ?>
copy that code, then bind the search_xtra column to the search_xtra2 select list. This will create code similar to:
<?php echo(isset($_POST['search_xtra3'])?$_POST['search_xtra3']:"" ?>
Place your cursor at the beginning off that code, and paste the code you copied previously so the full code looks like:
<?php echo(isset($_POST['search_xtra'])?$_POST['search_xtra']:"" ?><?php echo(isset($_POST['search_xtra2'])?$_POST['search_xtra2']:"" ?><?php echo(isset($_POST['search_xtra3'])?$_POST['search_xtra3']:"" ?>
You can then click OK to save your changes to the insert record behavior.