Another thing they don't mention is that when utility AC returns, each UPS in the chain tries to recharge, which can be a significant chunk of power. Any given

Another thing they don't mention is that when utility AC returns, each UPS in the chain tries to recharge, which can be a significant chunk of power. Any given UPS is sized so that its downstream load plus its own recharging power doesn't overdraw the circuit, but add two recharging powers and it's much more likely. I don't think this is an issue with double-conversion UPSs, since their input power is fixed by their rectifier size and they'll simply charge more slowly instead, but with standby type, it's very much a concern.