Here is a gross oversimplification of an intense setup I am working with.
table_1 (id, field)
table_2 (id, field, field)
info ( ???, field)
I would use a composite (multi-column) key.
CREATE TABLE INFO ( t1ID INT, t2ID INT, PRIMARY KEY (t1ID, t2ID) )
This way you can have t1ID and t2ID as foreign keys pointing to their respective tables as well.