I'm sure this won't get the least bit confusing. I mean, it's obvious that Susan is on the left--wait, no! Right! I meant right! Don't be confused forever nooooo~!

Seriously though, Nanase-Susan is shorter in panel three and wearing a shirt that is all one shade of grey, and Susan-Susan is taller in panel three with two shades of grey in her shirt and slightly different sleeves. Simple, right? Keeping this from being confusing will be easy, right?

...Yeah, I'm doomed. 

EDIT: I inadvertently made Susan significantly shorter in panel three than she's supposed to be. Her height actually matters beyond simple appearances at the moment, so I decided to fix it. Here's the original, and a handy height comparison between the original and the fixed version. As you can see, it's a significant difference.