Using more than one coupon codes?

I read it some days ago (don't ask where, I wish I could find it among my bookmarks) that you can use more than one coupon codes (I don't mean the giftcard now) when you purchase something, so the discounts just pile up pushing the price low as hell...

I have just tried it and it seems to accept only one code at a time...

I am dying to know if the theory above is real in practice too, any info about it, and if it really does work that way then how can one manage it? less