How to properly create composite primary keys – MYSQL
I would use a composite (multi-column) key. This way you can have t1ID and t2ID as foreign keys pointing to their respective tables as well.
composite-primary-key
Can I have multiple primary keys in a single table?
A Table can have a Composite Primary Key which is a primary key made from two or more columns. For example: Update: Here is a link with a more detailed description of composite primary keys.