What you need to find is the following code in newreply.php and newpost.php. You will want to make sure option value both is indicated to be selected in both files, like this :
if ( ( ($HTML == "On") && ($Markup == "On") ) || ( ($user['U_Status'] == "Administrator") || ($user['U_Status'] == "Moderator") ) ) {
$markupselect .= "<option value="both" selected>{$ubbt_lang['USE_BOTH']}</option>\n";