How to properly create composite primary keys - MYSQL

Here is a gross oversimplification of an intense setup I am working with. `table_1` and `table_2` both have auto-increment surrogate primary keys as the ID. `info` is a table that contains information about both `table_1` and `table_2`. table_1 (id, field) table_2 (id, field, field) info ( ???, field) I am trying to decided if I should make the primary key of `info` a composite of the IDs from `table_1` and `table_2`. If I were to do this, which of these makes most sense? *( in this example I am combining ID 11209 with ID 437 )* `INT(9)` 11209437 *(i can imagine why this is bad)* `VARCHAR (10)` 11209-437 `DECIMAL (10,4)` 11209.437 Or something else? Would this be fine to use this as the Primary Key on a MYSQL MYISAM DB?