How to UPS-protect this "dual source" setup?
As a former builder of UPS's for APC and other companies, the UPS has limited surge suppression, often just a single 20 mm MOV on the AC input. A serious and somewhat expensive surge suppressor will have many 40 mm MOV's in parallel on each phase and neutral to ground. Use the UPS for what it does best and buy separate surge devices for point-of use protection.

The more expensive suppressors will also have X and Y rated capacitors for basic noise filtering. Do NOT have the UPS run large motors as their lumpy sine-wave output can make the UPS fail.

Install suppressors before and after the UPS. They work best at their point of use. Every foot of wire to the suppressor adds another 100 volts peak to any surge that tries to get through.