I need to update two tables inside a single transaction. The individual queries look something like this:

1. INSERT INTO t1 (col1, col2) 
   VALUES (val1, val2) 
   ON DUPLICATE KEY 
      UPDATE col2 = val2;

If the above query causes an insert then I need to run the following statement on the second table:

2. INSERT INTO t2 (col1, col2) 
   VALUES (val1, val2) 
   ON DUPLICATE KEY  
      UPDATE col2 = col2 + val2;

otherwise,

3. UPDATE t2 
       SET col2 = col2 - old_val2 + val2 
     WHERE col1 = val1; 
   -- old_val2 is the value of 
      t1.col2 before it was updated

Right now I run a SELECT on t1 first, to determine whether statement 1 will cause an insert or update on t1. Then I run statement 1 and either of 2 and 3 inside a transaction. What are the ways in which I can do all of these inside one transaction itself?

The approach I was thinking of is the following:

UPDATE t2, t1 
   set t2.col2 = t2.col2 - t1.col2 
 WHERE t1.col1 = t2.col2 
   and t1.col1 = val1;

INSERT INTO t1 (col1, col2) 
VALUES (val1, val2) 
ON DUPLICATE KEY 
   UPDATE col2 = val2;

INSERT INTO t2, t1 (t2.col1, t2.col2) 
VALUES (t1.col1, t1.col2) 
ON DUPLICATE KEY 
   UPDATE t2.col2 = t2.col2 + t1.col2 
WHERE t1.col1 = t2.col2 
  and t1.col1 = val1;

Unfortunately, there's no multi-table INSERT... ON DUPLICATE KEY UPDATE in MySQL 5.0. What else could I do?