Creating a foreign key on the Union of two tables

I have 2 tables "A" and "B" in mySQL. They both have an attribute 'id'
which is unique between the 2 tables. I want the union of these 2
attributes to be a foreign key in a 3rd table "C". How do I specify
this constraing in sql (mySQL) in table C? And for complex reasons I
cannot merge the 2 tables A and B.

Thanks,

-Saad