PS6510 3 power supplies

The PS6510 has three power units and can only run on two problem is I have only two feeds into the rack, so I am going to put in an ATS but this has an 15ms switch over, the two feeds will come from Supply A and Supply B, then I have the ATS then that will also have a feed from Supply A and Supply B and will give me a Supply C, so if either A or B fails then I should run, but the issue is if A fails and the STS was actively using it then I would get Power unit 1 down, power unit 2 up and power unit 3 would have a 15ms blip and I want to make sure it does not take the device down, I would prefer an STS but the supplier screwed up and I need to get this online ASAP

Any information appreciated.