Okay so Apple has the book smarts and logic and Darling has the strategy and guile. Darling lies to everyone else and Apple lies to herself.
Look at this except from A Semi-Charming Kind of Life:
“Oh dear, you’re getting shiny,” Apple said. She reached into her jewelled purse and pulled out a powder puff. “Are you nervous?”
“A bit,” Darling admitted.
Apple blotted Darling’s nose. “Don’t worry. You are the perfect princess, and everyone knows it.”
Darling looks like everything Apple wants to be, but Darling herself doesn't want that at all! Apple respects and admires Darling for the very traits Darling knows are are an exhausting facade!
Apple is insecure about not being perfect (because of her hair, her glasses, her friendships with Rebels and sense of justice gradually turning her away from the legacy system). She earnestly does her best to meet her mother’s expectations, striving with everything she has. Meanwhile Darling seems to effortlessly meet her family’s expectations, but is actually weaving an elaborate web of deception that leaves her lonely and miserable. Neither feels safe to let anyone see beneath the mask, but for opposite reasons. God. I’m normal about them.
Apple hiding her glasses the way Darling hides her armour... Darling pretending to be more weak and helpless than she is, while Apple is literally walking around with impaired vision because she's internalised the royal culture's insane ableism. Both wanting to prove themselves as worthy and capable, but Apple as a leader, scholar and diplomat (traditional princess/queen strengths) and Darling as a knight and hero (rebellious and subversive), so they have to do it in the light and shadow, respectively.
The two fairest most perfect princesses, one desperately trying to stay atop her crumbling pedestal and the other climbing down from it under cover of darkness. So she can secretly do and be everything that Apple wants in a romantic partner. And then after Darling comes out - you know, as a Rebel - they have a magical True Love's Kiss on screen. Okay. Okay.