Double UPS on high load during power down

If you don't mind the downtime, just set the servers to power off as soon as (or very soon after) the power goes out. That way you won't deplete either battery.

I personally would connect one PS of dualPS servers to the UPS (either of those) and the other one directly to power grid. That will simple up your load balancing.

Best bet at loadbalancing would be to measure the power usage of each server at expected load and then dividing the load according to UPS specs.

A few questions to answer here...

As for the power supply distribution in dual PSU servers, the short answer is that it depends on the server manufacturer and system/BIOS configuration.

Please see: How do servers with redundant power supplies balance consumption?

With regard to using multiple UPS units, you should try to match the specs and load. I find it simpler to go with a single large UPS or multiple right-sized UPS units that serve logical groupings of equipment (e.g. Network rack, server rack).

I'm not a fan of the half UPS/half utility power approach. That introduces too many unknowns when you start mixing in equipment with single PSUs and trying to calculate loads.

Also see: Is there any value in protecting both rack power feeds with their own UPS?