Update

With the addition of the code I can see exatly what you mean. Similar to my inelegant hack below. Thanks++

I may be being obtuse but I don't quite see how. Essentially we have to 'join' the two tables so we can sum the counts (ie b occurs with a count of 1 in each of the original tables so need to have a count of 2 in the final table, whereas a and c retain their counts of 1)

With a merge we will just get 2 rows of b.

This works (it is a kind of a merge) but is not very efficient:

mysql> select * from tok1;
+-------+-------+
| token | count |
+-------+-------+
| a     |     1 |
| b     |     2 |
+-------+-------+
2 rows in set (0.00 sec)

mysql> select * from tok2;
+-------+-------+
| token | count |
+-------+-------+
| b     |     1 |
| c     |     4 |
+-------+-------+
2 rows in set (0.00 sec)

mysql>  insert into all_tok select token,count,0 from tok1 union selec
+t token,0,count from tok2;
Query OK, 4 rows affected (0.01 sec)
Records: 4  Duplicates: 0  Warnings: 0

mysql> select * from all_tok;
+-------+--------+--------+
| token | count1 | count2 |
+-------+--------+--------+
| a     |      1 |      0 |
| b     |      2 |      0 |
| b     |      0 |      1 |
| c     |      0 |      4 |
+-------+--------+--------+
4 rows in set (0.00 sec)

mysql> select token,sum(count1+count2) from all_tok group by token;
+-------+--------------------+
| token | sum(count1+count2) |
+-------+--------------------+
| a     |                  1 |
| b     |                  3 |
| c     |                  4 |
+-------+--------------------+
3 rows in set (0.00 sec)

mysql>
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We need the two columns for count so we have b,0,N and b,N,0 so the union does not obliterate one of the b's.