Can I use two UPSes to power the same device?
The "safe &amp; sane" approach is to replace the current PC &amp; switches with models that DO have redundant power supply inputs. If, in fact, there's anything like a reasonable NEED for that. People often get overly concerned with slapping UPSes on non-critical systems, in my experience. If the system was, in fact, critical, you would, in fact, have good reason to get one with multiple power supply inputs. Replacing the PC with a laptop that has a battery is another option for that piece.

Alternatively, you get a single UPS with multiple power supply inputs; if feeling creative you connect battery chargers to each phase and a true-sine inverter (or perhaps a cheaper MSW, depending on the loads you are feeding and what they think of MSW inputs - most computers don't mind) to the battery bank, but you waste power (generate heat) since the conversion in each direction is not 100% efficient - with one battery charger that is a "dual conversion" UPS, and with two it's a dual-input dual-conversion model. 

But really - how critical are the systems? If the system is critical, should you move it to a server farm where these things are Standard Operating Procedure rather than running it in your house? As a professional computer support person, I have quite a few systems where I'm quite happy with just a surge suppressor. Many modern systems shut down gracefully on power loss, and a surge suppressor does not need a constant stream of money to replace batteries as UPSes do. Few things are as annoying as a UPS that claims it can run for an hour when you ask its interface, but that dies in 5 minutes when subjected to real-world testing. [Or the "even more annoying" totally killed its battery, no indication, and then trips on a power event so short that non-UPS systems stay up, while the UPS shuts off instantly.] How often is one phase out for long enough that the UPS won't hold things up, while the other phase stays up? Should you just get much bigger UPS batteries instead? Should you just tell your PC to shut down gracefully after a minute or two of UPS operation to cover brief outages?