And a printer?

Plugged into two computers at once over USB? Never seen it.

What does the UPS use to send the signal?

Can you connect the NASes together via USB and generate the same signal via software?

(Or just attach the switch to the UPS so the network doesn't go down, and you can do it with SSH.)

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.