I have two identical 230V single-phase "line interactive" UPS'es, powering a few devices which only have one power supply each – half of the devices are connected to UPS 1, half to UPS 2. (The devices are Ethernet switches and a PC.)
What sort of connection or device would be needed to allow both UPSes to power the same device? (I have a feeling it would be easy for devices taking DC input, but not for those which only have AC in?)
And would it still be possible if the two UPSes were fed from outlets connected to different AC phases? (Which the room happens to have – the building has a standard European 3-phase supply, except it's a bit unreliable – and it's what prompted the question in the first place.)
I'm only looking for theoretical knowledge and not actually planning to do any such connections in reality. I don't want anything to be on fire.