A 400 amp transfer switch will fit the bill however I hope you are aware that you may have to change the numbers of conductors from the transfer switch to the panels ., it will have to be 4 conductor due some of the transfer switch do have main breaker in there and it will genrally rated for service entrance.several jobs. This one is 2-200 amp panels. almost all gas appliances so 20 K is good.
Other jobs I come across 2- 150amp panels and 22 or 24K is usually good
Yuh Four conductor 4/0 Alum SER cable is typical. unless your local code say different.You mean for 2- 200amp panels I'll be using 4-o to each panel?
You will have to make a choice either single 400 amp transfer switch or two 200 amp transfer switches either one will will work fine with it.Gilberts Corner, we're neighbors... I'm talking about a whole House transfer switch.
I purchased one of those today and have installed it several times. Good unit. ATF has two 200 amp mains.Just saw this... https://www.amazon.com/Briggs-Stratton-40396-20000-Watt-Generator/dp/B009GYCJIC
I'm looking for the Kolher one that GaryGood mentioned