Twilight likes doing things with her student/friend/sister/adopted daughter.

They should just have the two as ‘best friends’, just like Bon Bon and Lyra.

@Xonxt  
And so could Starlight, tbh. Thats not really a great criticism.

@Josh103
 
Yeah, they clearly didn’t have to try here. It was just the whole cast being a well oiled machine together handling the threat with the barest minumum amount of effort exerted

@Josh103  
I think so, too. I been suspecting that Twilight sort of wanted someone she can share her passion for magic with (and I mean just magic, not other geek stuff like science, books or history) and boy does she find that with Starlight, who not only is passionate about magic, but also as talented and as skilled as her (arguably).
 
The ELTSD episode has both Starlight and Twilight having fun doing magic together, Season7 premeire has Twilight implying that she and Starlight has been doing magic together to the point that Twilight’s run out of things to teach her.
 
Also, this scene is just a cute ‘Master-Pupil’,‘ObiWan-Anakin’ moment meant to showcase their relationship more than anything.
 
Look, Twilight is REALLY fond of Starlight for some reason, so I don’t see why they can’t do a combination attack just because they feel like doing it.

@Xonxt  
I believe it’s that, in-universe, Twilight likes to do magic with Starlight.

@Xonxt  
yeah

I’m pretty sure Twilight would’ve easily been able to do this herself… unless she got seriously nerfed with each passing season.