This is common and standard practice. It's quite possible that an alarm positioned at the other end of a house, with several closed doors in between, could easily fail to wake someone up. The goal is then to ensure that if one alarm triggers, they will all do so. Since they'll already likely be connected to the same electrical circuit anyway, this usually takes the form (in the UK at least) of a third core that connects the alarms together – and is a very simple system.
I won't speak to the "online" nature of any detectors, but I would expect standards mandate that they function fully and independently by design regardless of connectivity.
I won't speak to the "online" nature of any detectors, but I would expect standards mandate that they function fully and independently by design regardless of connectivity.