It’s possible that it’s not just Twilight’s and Sunset’s cutie mark, it’s possibly also Starlight’s in it, you can tell by looking at the purple side which clearly has shades of it, light for Starlight and dark for twilight.

@Jarkes  
they don’t even have the same shade of purple

@Background Pony #A0AA
 
It really doesn’t.

@Background Pony #026E  
Starlight is more Like Twilights best friend,

@Sunset’s Minion  
It’s a representation of this special and the new friendship between Starlight and Sunset.

@Background Pony #A0AA  
And why would Twilight even think to put Starlight’s cutie mark instead of her own?

Looks way more like Starlight’s cutie mark. Which is why Starlight giving her the book is no coincidence.

@Jarkes
 
Not much, that’s certain, but in terms of color it still manages to look more like the star in hers than it looks like Twilight’s.
 

 
And even then it’s stretching it – the shapes of the points are very different, Starlight’s have curved sides and these are straight-edged.
 
I could buy that it’s more of a symbolic or stylized representation of Twilight’s mark than a strictly realistic one, but Sunset’s cutie mark still looks… mostly like what it should be. The colors aren’t exactly right, but it’s immediately recognizable, which is the point. It’s that they got one half right while the other is completely off that baffles me.

It doesn’t even look like Starlight’s cutie mark either.

I’m going to say this twice, because I’m really confused by this: how does that look anything like Twilight’s cutie mark? There’s too many points, it’s in two colors rather than a single solid hue, and that purple and lilac are nothing like Twilight’s magenta.
 
I didn’t see Starlight’s cutie mark in this when I looked at it the first time, but seeing it brought up in the comments… yeah, I can definitely see how the purple side of this symbol looks more like her cutie mark than like Twilight’s.

@Jarkes  
You’d be surprised…

If the metaphor is true, I really hope they show more of Twilight and Sunset being best friends.
 
They likely won’t.

Well there best friends confirmed

That’s actually supposed to be half-Twilight?
 
The other half doesn’t look much like Twilight or Starlight’s cutie mark, but I always thought by the colors alone that it was supposed to be Starlight, and that Twilight set up the whole meeting between Sunset and Starlight by conveniently not being there.

…People seriously thought there was more to it than this?

Then you might wanna catch up with your color guy, because that looks way too much like Glimmer colors.

best friends shore we know what that really means

Its not that deep, fam.

Ok…just like Lyra and Bon Bon Are “best friends”

So…Sunset is Twilight’s best friend?

… that’s Twilight’s cutie mark?

@IceKitsune  
Yet the fanbase is gonna overanalyze it anyway as they usually do with other minor things in a cartoon about brightly colored horses that have horns and wings.

That probably would’ve been clearer if Twilight’s side actually LOOKED LIKE her cutie mark. It looks more like Starlight’s than Twilight’s.

The deepest of BEST FRIENDSHIP!!!

Uh huh. Sure.