I have 2 computers that we use for video editing they have 2 external hard drives. The thing is that in the past for some reasons the power went and the external hard drives got damaged. But I was wondering if it is better two have UPS of 300 wats or one UPS of 600 watts of similar quality.
As your machines (probably) won't have any PSU redundancy, I'd choose a single UPS that can support signalling to multiple machines over a network. When the UPS battery runs low it can shut down all the machines. By choosing a single larger capacity UPS, you benefit from economies of scale, and the UPS capacity is shared, so if only one PC is turned on, you'll have twice the runtime.
If you have machines with redundant PSUs I'd favour two UPS, with one PSU plugged into each UPS.
I would go with one bigger UPS. In fact, for two desktop machines with external drives, I would definitely go bigger than 600W, more like 1000W (in reality the really important figure is VA, not Watts, but let's not go there right now).
Plus: you want a UPS with either a USB interface or a serial interface or an ethernet interface, so that you can run the relevant monitoring software on at least one of these computers and shut them down in a controlled manner when the power fails and tha UPS batteries run down. Most modern UPSs support this, and the control software often has the ability to relay the shutdown command to clients installed on other machines via LAN.
It depends on the maximum load. If a system needs to pull (at least) 350 watts for more than a few seconds then the 300W UPS won't cut it, and you're better off with the 600W UPS.
On the other hand, having multiple UPSes means that even if you lose one UPS you can still work on the other machine.