What are the loads? Is the standby on the same property fed from the same utility service?
Any emergency loads like exit lights, smoke fans, fire fighter elevators, emergency lighting needs a seperate transfer switch, Fire pump needs a seperate transfer switch, Standby power for non emergency use like a computer or refridgerator requires a separate transfer switch.
From your post I am guessing this is just for standby power and no emergency systems? You can build a small emergency distribution too. 1 main to 2 distribution breakers or switches and 2 transfer switches to each unit with a normal and standby feed in with a protected feeder out to the regular panel. If you are not protecting all the circuits in each unit then you need to add another panel with just protected circuits installed. Remember the generator must be sized to carry all the load connected to it. So for example you have 100 amp out put and each space has a 100 amp panel the generator can only carry 50 amps for each so there would have to be a protected panel and only circuits up to 50 amps could be connected in a power failure in each unit. that requires a bunch more work and profit too.
So start with the loads and work back to the generator.
Have fun and get smarter at the same time.
