On professional equipment (like the ones we have at my work) you can just add batteries as you need.
1000v ups is ready up to 600w of use, so you should be fine, and no, you cant use 2 ups for the same psu.
And if the UPS turn off on a power blackout, its time to change the batterie, but it still offer surge protection.
I guess you can also just buy another external batterie backup to Plug your UPS in