Temporarily obtain 2 surge protecter power strips with switches. We need to turn off the power without breaking the ground circuit, so no yanking plugs out of the socket.
Obviously starting with both servers safely shut down, along with everything else you plan to plug into the UPS.
Plug all the power cords that will go to the UPS into surge protector 1. Plug the UPS input into surge protector 2
Make a (hopefully measured) educated guess on max power draw of everything plugged into surge protector 1. Bonus points if you have an accurate wattmeter to feed surge protector 1.
Leave the USB connection from the UPS going to Unraid 1. This is why you need surge protectors instead of yanking the plug. Very bad things can happen if the UPS ground isn't tied back to the PC when it loses power.
Fire up the UPS with nothing but the USB lead and power input connected.
Boot up all the equipment on surge protector 1. Start a non-correcting parity check on both Unraid servers, and have everything started and loaded like it would in a worst case unattended power outage. Read the power draw if you have meter of some sort.
Connect a similar draw non critical load to the output of the UPS. Good candidates are halogen work lights, incandescent lights if you have any around any more, space heaters on low, use your imagination to find enough load.
Now you are ready to start the actual testing portion. This should be done with observation only, resist the urge to manually intervene. You should have a way to watch the dashboard of both Unraids.
Turn off the surge protector feeding the UPS to start the "power failure".
Observe the loads connected to the UPS, look for flickering lights or fan speed variations on the heater, whatever you have connected.
Remember to turn off any tech not connected to UPS, to accurately simulate a power outage. Optionally for a more thorough test disconnect your outbound internet if you can do so safely, unless you are using cellular WAN, as a real power outage may drop the WAN outside of your control. Watch to see if you get notifications on the Unraid dashboards of a power failure. If everything is working to plan, Unraid 2 should start the shutdown process cleanly after 5 minutes. When it shuts down, you can adjust the load on the UPS to match, maybe turn off one of the lights connected. After the 10 minutes has elapsed, hopefully Unraid 1 starts a clean shutdown. When it's done, you can adjust the load on the UPS if you want. At this point if the UPS is still running the dummy loads, you can call it a success, depending on what you observed.
Other considerations.
After draining the batteries on a UPS, make sure you account for recharge time before depending on it for more backup, since a typical recharge rate is 10 to 20 times slower. If the UPS was running on battery for 15 minutes, allow at least 3 hours recharge time.
SLA UPS batteries, the most common type, get touchy about being drained more than 50%. Their capacity and lifetime is reduced the deeper the discharge, so try to stay in the top of the curve. If you are discharging too deeply, reduce the time on battery parameter.
Personally my secondary loads like client pc's, VM's, etc are all set to shut down after a minute or two of power outage. Keep in mind you can install apcupsd clients on any VM's hosted on Unraid to get them cleanly shut down prior to the main timeouts and reduce the shutdown time.
If during the test the batteries are drained before the timed shutdown is done, you need to upsize the UPS, or restructure to multiple UPS since you are aiming for less than 50% drain, and you can't shorten the delay meaningfully.
If the communication path between devices is broken or interrupted, the shutdown signal will be lost. Make sure all network infrastructure can outlast the full shutdown period.
I recommend NOT trying to have things automatically recover after a power loss event. Much better to manage the recovery hands on, watching and controlling, especially checking UPS battery condition to ensure enough capacity to handle another shutdown if power goes out again during the boot up process.