UPS - Two computers - How to get them to both shutdown when battery is low?

As Chopper3 writes, more expensive UPS' can have an option to attach networked shutdown software (for an additional cost).

For a free-of-charge implementation of networked power management you want APC UPS Daemon, apcupsd. Basically you install a manager instance of apcupsd on the PC that has the USB signalling cable attached, and install apcupsd clients on other PCs attached to the UPS power. The apcupsd instances use the LAN to communicate.

Apcupsd comes in both Unix and Windows versions, and the Windows versions have existed for quite some time, so I'd expect them to be stable enough. I don't know how easy it is to set up and configure on Windows. Take a look at it, and if you don't like it, there is always the option to buy a more capable UPS...

Remember, with networked UPS management the network switch needs to be on UPS power for the management software to work when power is out... :-)

Windows typically has good APC support built in, so you don't have to use PowerChute (at least starting with XP SP2 and SP3). If Windows is detecting your UPS under Control Panel -> Power Options (in XP), then look for an Alarms tab. If you can find this, you can figure an Alarm Action to run a program. In this case you might run PsShutdown from the sysinternals package to shutdown the remote machine when the battery level is at warning or critical.

I'm sorry, but I'm not at an XP machine with an APC UPS hooked up at the moment, so I'm not sure if these options will appear or not. I typically don't install the PowerChute software from APC on my XP (and above) clients because there's native support in Windows. (This is also why I standardized on APC)

In the long run, your best bet is two separate UPS's. More expensive, but it removes a single point of failure, and is less complicated than trying to coordinate a signaling system between the two machines (and it also reduces the load on each UPS).

You can't with that model, not with the software it comes with anyway.

If you had a 'Smart-UPS' model from APC you could use their PowerChute Network Shutdown software to achieve this.