As I said, I'm not sure this is *currently* supported, so I thought it was worth mentioning to at least keep the current features consistent.

That being said, if it's not, I think it should be worth opening another issue for it: it's [perfectly possible](http://stackoverflow.com/questions/953035/multiple-column-foreign-key-in-mysql) to [create a foreign key on two or more fields](http://dev.mysql.com/doc/refman/5.6/en/create-table-foreign-keys.html)!