Interesting. They do sell 400A CT sensors for the 15K, so it is probably OK to use theirs and they can say what ratio setting to use.

These are the 600A ones that they sell. They are big enough to run a number of large cables through them!
I don't remember the size of those wires, but for some reason I'm thinking maybe 2/0 copper or something like that. If you look closely at that picture, you will notice that on the CT it says 600A/300mA. That is a 2000:1 ratio, same as the smaller ones that come with the 12k or 15k units. Basically, all CT's from Sol-Ark are that ratio.
If 40kw is the max we can measure at 120/208V 3 phase, that gives us 40,000/208V=192.3/1.73 (for 3 phase)= 111A. So with a CT ratio of 2000:1 we go 111A/2000 = 0.0555A. So, the Sol-Ark circuitry maxes out at 0.0555A of current..... Theoretically, I would think paralleling 2 CT's should just add the mA together, and should work. But you will still ultimately be limited to 111A of total current with that CT ratio.
I think (someone correct me if I'm wrong here.) that using a CT ratio of, say, 6000:1 would yield 1/3 the mA of current at any given A flow through the CT's, therefore tripling the A limit that is measurable to 111x3=333A. However, I have yet to get the blessing from Sol-Ark to go ahead and try it.... They either don't care enough about my specific job where I have this issue, (what's one fish in the river, right..???) or they don't actually know if it'll work because they haven't tried it themselves and therefore they won't tell someone it's ok to try it.
I do question why anyone would design an inverter with 200A transfer switch and external CT's that can't measure 200A........... I realize that if you just run a full 200A service through the inverter, it can supposedly measure up to 200A on the internal CT's, but what about Limited to Home scenarios...??? After all, it is designed to be able to do Limited to Home...... I fully assumed that external CT's would actually give a higher measurable amp limit than the internal ones. Sol-Ark recommends using external CT's whenever you have more than one inverter in a system.... So let's say we would stack 8 of the 15K's on one system........ You would never, never, never......... (did I say never????) have a scenario like that with only 111A of power feeding into the inverters! Never!