They are released as 2 separate series by Netflix:

• Dahmer - https://www.netflix.com/title/81287562
  
• Monsters - https://www.netflix.com/title/81665094

Consequently, they shouldn't be merged.

Click on the link to the Monsters (https://www.netflix.com/title/81665094) and scroll down the page. You will see the text "The second installment of the "Monster" anthology series from co-creators Ryan Murphy and Ian Brennan."

Even with that sentence, they are released as 2 separate series.

You are right, but on other platforms such as IMDb and Rotten Tomatoes, the show has also been labeled as the second season of an anthology, and it would seem logical to refer to it in this way, especially since Netflix itself has confirmed this.

@Kvadrat said:

but on other platforms such as IMDb and Rotten Tomatoes, the show has also been labeled as the second season of an anthology, and it would seem logical to refer to it in this way,

Each platform has its own set of rules.

especially since Netflix itself has confirmed this.

Netflix has released them as 2 separate series. So, we need to do the same here.