I bought some wireless smoke alarms like this for my elderly parents. My sister and her husband live upstairs and my parents live downstairs. I bought these at Home Depot, I believe. Just set some switches inside both units to pick a particular combination for your house, and whenever one goes off, the other one does, too. A bit more expensive, but worth it. And battery powered, so no need to do any wiring work.

Another application for this in my house is between smoke detectors far from our bedroom or in places where the sound won't travel, like the attic space over our garage. It's very hard to hear sounds from the garage in the house, but if that smoke detector goes off, the detectors in the living areas go off.