Both glands have ducts outwards, but only the mandibular gland produces pheremones (but it's more complicated than that, depending on whether she is a queen or a worker, and the age of the worker). The pheremone gland you're probably most familiar with and have seen a lot in action is the Nasanov gland at the tip of the worker's abdomen - but there are quite a few more.
I highly recommend Celia Davis, 'The Honey Bee Inside Out'. That book has helped me enormously to understand the working of bees a bit better - but I have a lot more to learn.
Kitta
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