How to properly create composite primary keys – MYSQL

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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?

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