Does it depend on which device is acting as the host? If it's the UPS then should maybe work, if it's the NAS then you'll have two hosts on one USB and things will cry.

Yeah, I understand why you have a UPS, I just didn't get what you were asking. It does look like the common way of doing this is to have one NAS with the USB connection as a primary (Network UPS Server) and then use one of a variety of tools to set the new NAS as a slave to it. But this relies on the network being available - which in your case you won't have. 

I suppose it depends on whether the UPS does some sort of pairing via USB and only allows one pair.

This.

In my experience, the UPS will be a downstream device (think printer, thumbstick, etc.) and you will optionally have some software (usually built into the OS) that detects the change in power source from the UPS and displays that to the user or logged somewhere.

It's unlikely to work the other way around.

Glad I'm not going crazy.

The NAS doesn't talk to the UPS, it's the other way around. The UPS does all the talking, the attached NAS/PC/whatever just listens and reacts to whatever the UPS is telling it. On USB. That's why I suppose a hub might work.

Much like a flatbed scanner talks to a computer and sends the image to the computer and not the other way around, you can't connect the scanner to two computers simultaneously.
Or a mouse, or a keyboard.

It'll still be two way communication. Host (NAS) and Device (UPS).
I'd love to see it work, but my guess is it won't.

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.