Switchboard Closed Transition Return to Normal Power Source

Utility feed is through two different transformers that means these two infeeds to the switchboards cannot be paralled and further the generator(s) cannot be synchronized with both the utility infeeds at the same time since two different transformers get involved.

I forgot to mention only 1 utility feed will be on at a time.

First of all, what is your question? What is the problem you are facing?

The Incoming LVPCBs (Low Voltage Power Circuit Breakers, I understand) need not be rated for the combined fault current of both the sources. They need to be rated for the fault current delivery capability of the respective source only.

But, the Panelboard bus bars MUST be rated for the combined fault current - both from the thermal withstand point of view as well as from the elctro-dynamic wthstand point of view.

Momentary paralleling should not be a problem in most cases. But, please ensure whether your local power supply authorities would permit you to do so. Particularly in Low Voltage, this is objected to by most authorities. Even in HT it needs a special permission from the authorities, involving a thorough review of the power and control schematics.