if there is a single method called the "Type A and Type B" method, then it is singular. if there is a method called the "Type A" method and another called the "Type B" method, then together they are plural.

Nouns are singular and plural depending on their meaning in themselves, not on the adjectives that precede them. e.g. a black car, two white cats, a black-and-white television, two black-and-white televisions. Note that the compound adjective "black-and-white" described the television in one way, its type. It was not a colour TV. The rules of punctuation usually require hyphens (-) in compound adjectives. However, this rule is widely ignored and occasionally, this produces confusion. I wonder if this is what has perplexed you.

I will change the example a little because “in” does not work for me with method: - the effect was confirmed using statistical and non-statistical methods - the effect was confirmed using the Potyomkin and the Oblomov method - the effect was confirmed using both the Oblomov and various non statistical methods I would use A and B methods if there are multiple methods in each category. I would use method if there is only one in each category. The O and P methods sounds wrong to me if there is only one O and one P method. Finally, the same would apply for other nouns, too, (e.g. approach would have behaved the same way