My suggestion comes from personal experience, we had a rack where there were two UPSs and one "direct" mains PDU. Some of the servers had both inputs on UPSs (separate) and some servers (the ones I commissioned!) had one input in to a UPS and the other into the "direct" mains PDU.
Mains service failed, so all servers stayed "up" as they had at least one input on the UPSs.
Generator kicks in, so "direct" mains is restored, so far, so good, everyone is happy
Mains service is restored, however during this process the circuit breaker protecting the 16Amp circuit to the UPSs "throws", as a result the UPSs aren't being supplied with mains.
Everything is fine for 45 minutes until the UPSs run dry.
So, servers that had both input off (separate!) UPSs - down.
Servers with one input off UPS and one off mains - up.
So if you have the budget/space/facilities try and split your server power input strategy to mitigate single point of failure. In that case, the SPF was that both UPSs were off the same mains feed and therefore an SPF existed at the circuit breaker.
We were fortunate that there were other separate 16Amp feeds in to that room so the servers (and switches) ended up with one in the mains and one in the UPSs.