tldr; Get a large enough single UPS to treat your server as a single entity all on the same UPS. You can use the small UPS to keep your modem, router, and switches running.
Keep in mind that if you have the computer and the DS on separate supplies, you must ensure that the DS is always powered up before the OS boots, and powered down after the OS has completed a clean shutdown. How you manage that is up to you, but be mindful of what happens during all plausible scenarios.
Normal startup and shutdown
All power loss scenarios assume you aren't around to manage things.
Power loss short term, power restored in less than a minute
Power loss long term, power restored after 10 minutes, subsequent power loss within a minute or 2.
Power returns after several hours.
I'd advise planning for the array to stay down until you can supervise and manage the startup, there are just too many moving parts and interconnected events to try to coordinate.