Another important point is that an UPS can produce huge surge currents, for example when switching from battery to utility. The upstream load goes from zero to the sum of both the protected load and the UPS charging its battery.
When daisy-chaining an UPS to another, the downstream UPS can easily overload and trip the upstream UPS because of that.
When daisy-chaining an UPS to another, the downstream UPS can easily overload and trip the upstream UPS because of that.