I have not yet extended my visa but if I understand corrcectly one of the requirements is to have 400k in the bank 2 months before the date

I also understand the bank has to provide a letter confirming it came from abroad

The question is does that have to be one transfer or could it be a combination of say 2 x 200k

There is no requirement that it has to come from abroad.

Bank letter verifying the balance and up to date bank book is needed.

It could be 1, 2, 3, 4, etc., transfers as long as the total bank account balance reaches at least 400,000 Baht two months before your application date "and" you keep the balance at or above 400,000 Baht the entire two months before your application date. And as mentioned about, the money does not have to come from abroad; it just needs to be money in the bank of at least 400,000 Baht for the entire two months before the application date.

Seasoning period is 2 months for first application, 3 months for subsequent renewals.