Two-book series/duologies seem to be gaining popularity in YA! Check out some of these duets! Except Treatment is a trilogy not a duet.
The Lunar Chronicles . Absolutely one of my favorites ! It's a clash of Disney princesses, Indiana Jones and Star Wars! Must read Marissa Meyer
Teh Chronicles of Narnia - C S Lewis
You have every book in the series proudly displayed in your house. 19 Undeniable Signs You're Obsessed With "The Mortal Instruments" Series
20 series to read if you loved divergent
Snow Like Ashes is one of my favorite series!