So the final result could be calculated like so?
$query = mysql_query('SELECT COUNT( t1.column1 ) AS t1count, COUNT( t2.column2 ) AS t2count
FROM table1 AS t1, table2 AS t2') or die(mysql_error());
$totalrows = (int)(mysql_result($query, 0, 't1count')) + (int)(mysql_result($query, 0, 't2count'));