No, you can't and shouldn't daisy chain (or serialise) a UPS by plugging in another UPS.
Run them side by side (in parallel as I think you're calling it) and make them power completely different devices/servers.
Or, better still, buy a much larger capacity UPS.
I quote the APC website which sums up the reasons why not beautifully.
Q: Is it possible to daisy-chain UPS systems together?
A: APC does not
recommend that you daisy-chain two or more UPSs together. Each unit
should be plugged directly into a properly grounded wall outlet for
optimum surge protection. We do not recommend daisy-chaining UPS
systems for the following reasons:
Daisy-chained UPSs do not provide any extra surge protection. A UPS is designed to remove any possibility of a surge reaching the output
receptacles. In the event of a surge strong enough to damage
equipment, the first UPS in the chain would sacrifice itself to
protect the load. This would mean that power would be removed from the
second UPS in the chain, forcing the unit into battery operation.
Whenever connecting a second UPS to another UPS, the chance of overloading the first one is greatly increased. The number of
receptacles in a UPS is restricted by the power limitations of the UPS
itself. Even if the number of receptacles was increased, the overall
Watt capacity of the first UPS would remain unchanged. The capability
of the second UPS would be inversely affected by that of the first
unit. Therefore, the overall Watt capacity of the configuration is no
greater than that of the first UPS in line.
In most cases, daisy-chaining UPSs does not allow for extra runtime. If you are using a UPS that outputs a step-approximated sine
wave when running on battery, as soon as the first UPS goes to battery
operation, the second one will also do so because it will see the
step-approximated sine wave as distorted or bad power. Both UPSs will
discharge together and will not provide any extra runtime to the load.