Kevin...........thank you for the reply and clarification.
1) Is the gen-set a portable or a standby?
2) Since the gen-set is a 6KW, I assume that you are only looking to back-up power to critical circuits in the house, possibly the fridge, well, furnace, a sump pump (if applicable), lighting circuit?
3) 12 AWG stranded copper is only rated at 20 amps and not 30 amps. For a 30 amp rating, the feeder conductors need to be sized as a minimum of 10 AWG Stranded copper or possibly 8 AWG if the distance is exceeded due to any voltage drop from the conductors. If the main panel has a double pole 30 amp feeder breaker and those conductors are feeding into a 30 amp main lug panel, the conductors need to be minimally sized as 10 AWG Stranded copper.
4) In a generator installation ,there is no need for having multiple Xfer switches and I would not recommend such an installation. A gen-set Xfer is only installed at the main service panel and NOT co-located both at the main and at a sub-panel running dual Xfer switches. The intent of an Xfer switch is to prevent back-feed power to the utility grid from the main service panel. The main service entrance, ie... main panel is the location where the gen-set power needs to stop the current flow out to the grid.
5) Many Multi-story commercial buildings and factories all have numerous sub-panels installed throughout the various floors or areas of a building using (1) one generator, yet they only have (1) one Xfer switch located at the main switchgear or main panel. They do not have multiple Xfer switches installed. A single family dwelling is no different... just on a smaller scale.
Hope this helps............If you have any other questions, just let me know.
If no other questions, don't forget to rate me before you log off..............Thanks..........Kevin!