Using TimeMachine they can be, under one of two methods- Manually swapping between computer, or attaching the drive directly to Apple' Airport Extreme and sharing the drive. Apple also sells the "Time Capsule", which does WiFi back up and routing, but is very spendy.
Apple - AirPort Extreme Base Station - White
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Apple - AirPort® Time Capsule® 3TB Wireless Hard Drive & 802.11ac Wi-Fi Base Station - White
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