I don't know, I guess whatever bus is in use or it blindly fires away on them all. There might be a way to do what you suggest (chain the devices via usb), but the custom code angle would be beyond my ken. I'll sniff around here http://www.apcupsd.org

Indeed, but the USB spec doesn't allow what you want to do, even with a hub which itself counts as being a downstream device.

While you can have multiple downstream devices connected to one upstream you cannot have multiple upstream devices connected to one downstream. There are workarounds to this, like USB switches but these still only allow one upstream device. Pushing a button on the switch disconnects one upstream device and connects the other. The two upstream devices are never connected at the same time.

I wouldn't pursue what you're thinking. But what I would look into is having something as an intermediary device (think Raspberry Pi) that can be connected to the NAS and receive it's notifications and then communicate them over ethernet to the two or more upstream devices.

OK thanks.