I'm chiming in on this late, but the photograph is from Hutton's Onteora
photo album, and he identifies them as "Miss Susie & Clara Clemens." in the
margin.
The order is off, if he meant to identify them from left to right.

A comparison of the Bryn Mawr photo of Susy in Mallory's zip with the gal
on the right, along with this image on MTPO, where Susy is in the window
seat and Clara on the floor:

http://www.marktwainproject.org/xtf/search?keyword=PH00131&x=0&y=0&fieldList=text+date+artist+depicted+pubnote+collno+captionnote+place+repository+title-uniform+identifier&style=mtp&brand=default&sort=date&category=images

makes a good case for Susy on the right. Hutton's identification of Clara
should cover the gal on the left.

I can't post images, but if anyone is interested I composed three
juxtapositions which I can email. There is an image of Clara in the P & P
owned by the Hartford House that comes close enough to looking like the gal
in the left, to my eyes.

Leslie Myrick