Young Writers Society

In my current series I plan to write five books in total. However, after looking over the plots, I feel books one and two are completely unnecessary and their plots can be altered without impacting the story at all.
Since books three through five focus on a new set of characters, the history of books one and two (which take place twenty years prior) can be explained in a prologue, through some explanation from a character or hinted at throughout the trilogy.
Also, since this is adding two more books to the series, I feel it is becoming somewhat of an over kill as this is the series so far:
Story Arc 1
Book I
Book II
Story Arc 2
Book III
Book IV
Book V
Story Arc 3
Manga I
Manga II
Other
Prequel Book (made up of multiple stories similar to World War Z)
Guide to the Races and worlds within the story line.

Do you think books one and two should be scraped?

I think you should tell the story in whatever length is necessary to tell the whole story!

Which sounds kind of "well duh, I know that" of course. But it's true! Listen to your story, and give it the space it needs without dragging it out unnecessarily to fit some predetermined number of books. Stories change and evolve as you write them, too, so just let it flow! And figure out where to break things up later.

So if you feel books one and two are irrelevant and not interesting enough for their own stories, I vote scrap!

If you feel like part of the story is completely unnecessary, then it really is best to cut it out! Readers usually get pretty annoyed when stuff is unnecessary, and you'll probably get bored writing it.

If it's interesting enough, then I wouldn't put it in part of the same series but instead make them stand alone novels that are in the same world (which is more common to do than you think!) so readers can pick up either the trilogy or duet and appreciate them, but gain extra knowledge about the world if they have read both series.

The author I know who does this is Tamora Pierce. She's got two worlds and writes basically all her stories within one or the other. You learn all the characters over time and while you can pick up any book at any time, you get a sense of watching the characters grow over time as characters progress.