Parallel UPS system is not directly supported. As a work around, it is OK to model parallel UPSs as one UPS as long it is an N, or N+1 setup.
For an N+1 setup, make sure that the modeled UPS has an internal redundancy of N+1 and that the UPS Capacity is the 'N' part. For example, 3 identical 50KW UPSs in N+1 would give a capacity of 100KW because the third UPS is "standby."
If you have Data Center Expert UPS devices, you can associate all of them on the one UPS model object and you will get all alarms. The down side is that impact might not handle the situation correctly, reporting loss of power instead of loss of redundancy.