It seems that DB.php cannot handle different connections for SQL Server. I noticed that you fixed this for MySQL. 

In mysql.php you have this:
 if ($dsninfo['database']) {
            // fix to allow calls to different databases in 
            the same script
            if (empty($GLOBALS['_DB_mysql_changed_database'])) {
                $this->_changed_db = false;
                if (!@mysql_select_db($dsninfo['database'], $conn)) {
                    return $this->raiseError(DB_ERROR_NODBSELECTED, null, null, null, mysql_error($conn));
                }
            } else {
                $GLOBALS['_DB_mysql_changed_database'] = true;
                $this->_changed_db = $dsninfo['database'];
            }
        }

And in mssql.php it's only this:

        if ($dsninfo['database']) {
            if (!@mssql_select_db($dsninfo['database'], $conn)) {
                return $this->raiseError(DB_ERROR_NODBSELECTED, null, null, null, mssql_get_last_message());
            }
        }


In my scripts I noticed that it handles only the last connection made and ignores queries with the other objects created to access database.