Question about SFD / ADU w/ Stand by generator

[pachome]

First post. Hope picture attached okay.

I have a generator setup I want to install. I've mostly installed Generac for simple 200amp residential services. I did one for a 320amp service with two panels. It had a standard UG non fused 320A meter base. I installed two 200amp Generac/Siemens gen ready panels w/ built-in ATS. Installed a 22KW generator. Ran the generator 100A feeder wire to a exterior pvc junction box. Split from that j-box, and took 100amp runs to each of those 200A generac panels to feed the 100A generator supply breaker built into it.

So my understanding is that when the power would go out, the breakers in each panel would disconnect from utility only allowing return power from the generator to go down the neutral. Power comes back on, the generator disconnects power, stopping any generator 120v return power that could go down the neutral. Utility power is restored and now only utility return power could be going down the neutral.
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So my new project. Its one big square house with an ADU. The ADU is pretty much the bottom floor. My one concern is, could I potentially be sending utility 120v return power and generator return power down the shared utility neutral at the same time when the utility power is restored.

If you look at my attachment you will see that its a UG 400amp rated dual meter pack w/ two meter sockets. Meter for the house would be breakered at 200amps. Meter for the ADU would be breakered at 125amps. So when generator is running I assume there is return 120V power coming back on the utility neutral. But when power is restored the second 125A meter would be live and could it send power down the utility neutral before the ATS panel on the main house kicks back over to utility feed?

If you happen to see anything else wrong with the illustration I made I gladly accept any comments and or criticism.

I do appreciate your help on this. Very much so.

 

[pachome]

Trying different pic file types. Not sure if the original opens large enough to view

 

[pachome]

Okay my pic is coming up small. I tried it as attachment, and insert image.

Tried Jpeg, BMP, PNG.

I'm really sorry, completely new to this site. Any suggestions so that I can get it to post clearer?

 

[ActionDave]

Your first pictures at the top of your OP are fine. They show up as a thumbnail that one can click on to get a full sized image.

When you have a non Separately Derived System set up with a generator there is no danger of backfeed if the transfer switch is wired properly even with any number of neutrals tied together.

Think of the utility as one car battery and the generator as another car battery. Take both battery negatives and tie them together with the neutrals on a common buss. Next tie the positives to their respective lugs in the transfer switch. As long as only one battery at a time is supplying power to the house then there is no complete circuit path into or out of the battery with the unconnected positive.